Exposed

'Love & Hip Hop' star Teairra Mari said her ex-bf leaked a sex video as revenge for breaking up with him ... so she's suing him and 50 Cent for reposting a sexually explicit image.

Teairra -- flanked by her attorneys, Lisa Bloom and Walter Mosley -- broke down in tears during an emotional news conference Thursday to announce the suit. Teairra said the ugly episode was triggered after she broke up with Akbar Abdul-Ahad after finding out he was cheating on her.

The reality star claims Akbar had her Instagram password and posted a video and an obscene photo of her as payback and to humiliate her. Teairra claims Fiddy, Akbar's close friend, then reposted an image to his 18 million followers before it was taken down.

Lisa said they'll file a police report later Thursday along with the suit. She added, "We have a significant piece of evidence that points to [Akbar]." AAA denies he was behind the act.

Kordell Stewart says newly surfaced video purportedly showing the ex-NFL star in the nude is real -- but it was stolen -- and he's pointing the finger at his reality star ex-wife.

TMZ Sports spoke with the former Pittsburgh Steelers QB who says he's outraged that the video went public because he had made it specifically for a woman he was dating at the time.

Stewart says he's already contacted his lawyers and plans to rain down hell on whoever released it.

Kordell says his #1 suspect is his ex-wife, Porsha Williams -- claiming she had sent him text messages warning him to "prepare for war" ... and he believes she was referring to the tape.

As for reports that Kordell had made the video for another man, the ex-NFL star tells us it's "bull****" -- and says that, while he respects gay people, that's just not who he is.

Justin Bieber is wise enough to know when a gift needs givin' ... but he didn't have a box -- so he covered his junk with a guitar and jammed out a naked serenade for one lucky lady fan -- HIS GRANDMA!

TMZ has obtained photos of a completely nude Biebs strumming away on his big wooden instrument ... at his grandmother's home in Toronto during Thanksgiving back in November 2012.

And why, you ask, did Bieber go ass naked for g-ma?

Our sources tell us JB had been staying at his granny's house during the holiday ... and slept in a little too late on Thanksgiving morning.

Bieber finally woke up after hearing family and friends stirring around the house -- so he thought he'd prank 'em ... by grabbing his guitar and playing a naked set a la Jenny from "Forrest Gump" ('memba that?).

We're told birthday-suited Bieber went right up to his grandma and started belting out some impromptu lyrics ... like, "I Loooove you grandmaaaa ... how are youuuu ... helloooo grandma."

We're told G thought it was pretty funny -- and everyone in the home was cracking up -- but granny begged the pop star to get back in his room to put some damn clothes on, stat.

Thanksgiving -- they go balls out.