Chris Brown & Rihanna

WWE heavyweight champion CM Punk is lashing back at Chris Brown ... claiming the singer isn't just a liar ... but a total "coward" who never truly "paid" for what he did to Rihanna.

Punk released a video statement moments ago ... in response to Brown's allegations that he's a steroid-abuser with an inadequate penis.

In his video, Punk starts out by saying he's "a proud life long drug free straight edge individual" ... and then spends the next minute BLASTING Brown.

"Chris and I come from different worlds. I don't have an assistant. I don't have a body guard. I don't have a manager or a PR person telling me to delete tweets like Chris does, and I don't hit women. Period."

He adds, "[Chris] has made the wrong accusations to the wrong person because I can defend myself."

Punk then challenges Brown to a no-holds barred fight for charity ... and says, "I will choke you out and I will make you feel as weak and as powerless and scared and alone as any woman who has the misfortune of knowing a sad cowardly little boy such as yourself."

Your move, Chris.

5:16 PM PT -- Chris Brown responded:

Rihanna and Chris Brown have reconciled ... at least professionally ... because the former couple have collaborated on 2 brand new songs.

It's only been 3 years since Brown pummeled Rihanna's face during a late night argument in L.A. ... but the singer and her ex-con ex-BF have each released songs featuring the other's voice. One is a remix of Rihanna's song, "Birthday Cake" and the other is a new version of Chris Brown's, "Turn Up the Music (below)."

Both songs hit the Internet this week ... and both Rihanna and Brown have been pushing the songs on Twitter.

The songs seem to prove the rumors ... that Rihanna has forgiven Brown and is willing to move on. Earlier this week, Brown tweeted Rihanna "Happy Birthday" ... and Rihanna actually responded, "Thanks."

Rihanna is currently in London (see photo above) ... and Chris Brown was most recently seen in Miami.

Lost in all the Chris Brown/Rihanna possible reunion madness is the fact that CB has a serious girlfriend -- and sources close to her tell TMZ the two of them are doing quite well in their relationship ... thank you very much!

Sources close to Karrueche Tran tell TMZ she is unfazed by the recent rash of RiRi rumors. In fact, we're told CB and KT are now actually living together.

As for CB showing up to Rihanna's birthday party, we're told that didn't bother Karrueche either ... because he came home to her that night and whisked her to Miami the next day to celebrate Valentine's Day.

According to our sources, Chris professes his love to Karrueche daily and he has no plans to leave her for Rihanna.

Chris Brown was so paranoid about keeping his attendance at Rihanna's birthday party a big fat secret -- he actually tried to FORCE partygoers to sign confidentiality agreements ... TMZ has learned.

Sources tell us, Chris refused to enter the private party Monday night at the famous Hearst Mansion in Beverly Hills until everyone signed an NDA -- sending his henchmen in beforehand to gather every partygoer's signature.

We're told although they didn't get EVERYONE to sign -- Chris eventually walked in ... and spent a huge chunk of the night nuzzling up against Rihanna.

According to sources, the two got real intimate -- sitting next to each other, talking, and holding hands ... all perfectly legal ever since the restraining order was lifted.

As for the rest of the party -- we're told it was jam-packed with celebs including Katy Perry, Bruno Mars, and Chris Martin (no Gwyneth).

Rihanna's cake was pretty badass too -- a giant spliff ... with a small Rihanna made out of icing, riding it like a horse.

Eat your heart out, Miley.

Chris' rep tells TMZ, confidentiality agreements were distributed only to workers at the party and anyone who was caught videotaping/snapping a pic of CB.

Chris Brown and Rihanna are NOT back together, despite rampant rumors to the contrary ... Breezy's rep tells TMZ.

As we previously reported, Chris and Rihanna were spotted out at the same West Hollywood nightclub Sunday night. They left separately, but there was lots of speculation that the two had gotten back together.

But that's not true -- according to Chris' rep, the singer was with his actual girlfriend last night ... an aspiring model named Karrueche Tran ... not with Rihanna.

Rihanna and Chris Brown partied at the same club last night, fueling growing rumors they're back together.

The two singers left Greystone Manor -- a West Hollywood club -- separately, but they were both definitely there at the same time.

The protective order -- requiring that they have no contact with each other after Brown beat Rihanna -- was lifted a year ago.

Some "lucky" BET viewers were chosen to present the viewer's choice award at the show last night ... but there was a HUGE mistake ... because someone put the wrong name on the teleprompter.

So the lucky fan read the winner as "Chris Brown" ... but quickly recanted and said, "I'm sorry, the winner is Rihanna." But that wasn't the case either ...

Despite the fact that Rihanna's name appeared on the video screen as the winner ... one of the presenters chimed in for a THIRD time and announced Drake as the winner!!! Drake then appeared on stage to collect his trophy.

Drake gave a short speech and described the whole thing as awkward. Understatement.

After the awards, a BET rep announced that Brown was the correct winner -- and chalked the whole thing up to a "human error" on the part of BET producers.

We were hunting around online when we noticed their Twitter pages reveal that both are not only following each other, they added each other very recently. We grabbed the screen shots this AM.

A Rihanna fan noticed what we noticed and asked her if it meant they were getting back together -- Rihanna tweeted back, "its fu**in twitter, not the alter! calm down."

A judge lifted the restraining order prohibiting them from having contact, so it's all on the up and up.

Though Robin Roberts says Brown was aware she was going to ask about Rihanna, Chris told a different story on last night's "106 & Park" on BET -- where he said the talking points approved by his people were all about "positivity" and his new album.

Brown says he was "thrown off" by Robin's line of questioning -- and said she only brought up the Rihanna incident in an effort to "exploit me."

The singer says he then exploded backstage "to release the anger that I had inside me," adding that "I got very emotional and I apologize for that."

For their part, ABC News this morning said "Chris Brown was invited on 'Good Morning America' to perform and be interviewed. There were no talking points offered."

ABC sources tell TMZ ... Brown performed and was interviewed by Robin Roberts live from the Times Square studio when she began asking about the infamous incident.

Brown tried to redirect the questions to focus on his album -- but Roberts continued to ask about Brown's legal issues stemming from the Rihanna incident.

We're told after the interview, Brown freaked out, storming into his dressing room and screaming so loud, the people in hair and makeup became alarmed and called security.

We're told Brown was out of control, and one source present tells us he smashed a window in his dressing room, and the glass shattered and some shards fell onto 43rd and Broadway. ABC security tells TMZ ... the window was shattered with a chair.

We're told by the time security rushed the area, Brown had ripped off his shirt and left the building, blowing off another performance he was supposed to do for the ABC website.

And sources say ... on his way out of the building, Brown confronted a segment producer, got in his face and stared him down. People from the show got in between Chris and the producer to defuse the situation.

Remember -- Brown is on probation for the felony beating and he's required to obey all laws.

UPDATE 6:49 AM PT: Brown just tweeted, "I'm so over people bringing this past s**t up!!! Yet we praise Charlie sheen and other celebs for there bulls**t."

UPDATE 6:59 AM PT: Brown has already deleted the previous tweet ... and added a new one that reads, "All my fans!!! This album is for you and only you!!! I'm so tired of everyone else!! Honestly!! I love team breezy!!"

UPDATE 7:48 AM PT: Glass workers just showed up to fix the window.

TMZ obtained audio near the dressing room where Chris allegedly took a chair and smashed a window, sending shards of glass onto 43rd Street. During the conversation between the staffer and a glass repairman, the staffer says "I have a police officer coming just to take a look at it before we do any work."

Despite the conversation, NYPD sources tell us ... none of their officers were dispatched to the scene.

Sources tell TMZ ... Roberts insists both she and other "GMA" staff members spoke with Chris before the interview and asked if it would be OK to fire off "a few questions" about the Rihanna incident ... and Chris approved.

As TMZ first reported, Brown flipped out after the interview -- and apparently smashed a window in his dressing room.

Robin is telling people at the show she did NOT try to set him up ... and insists, "I'm pulling for the guy."

As for the broken window, sources say no one in the studio saw Chris throw the chair into the window ... but he was definitely in the dressing room when the window was smashed.

According to MediaTakeOut.com -- the website that posted the pic -- the photo was taken shortly after the night of the 2009 incident.

Brown had told investigators Rihanna had hit him first that night ... and scratched up his face.

Ultimately, it didn't matter -- and Brown plead guilty to assault.

MediaTakeOut.com has published photos of Rihanna which appear to have been taken at a hospital back in 2009 ... following the brutal attack.

No comment from Rihanna's camp ... at least so far.

Shortly after the incident, TMZ published a photo of Rihanna's injuries. Brown eventually plead guilty to assaulting Rihanna and was ordered to stay away from the singer.

However, just this week, a judge agreed to allow Brown to once again have contact with his ex-girlfriend.

E! first reported Rihanna was interested in changing the restraining order. Her attorney, Donald Etra, tells TMZ, "Rihanna does not object" to reducing the order to a "do not annoy" -- meaning the exes could have contact as long as Chris doesn't harass or molest Rihanna.

Chris initially asked for the reduction ... but the court needed to know how Rihanna felt before ruling.

The order has not been changed yet -- so there won't be a Grammy night reunion this weekend -- but Rihanna's change of heart hints there could be one in the near future.