Chris Brown & Rihanna Get Blunt at Grammys Afterparty

Rihanna and Chris Brown's new romance has already gone to pot -- the couple celebrated CB's zero Grammy wins by hitting up a Hollywood nightclub last night ... and lighting up what appears to be a joint.

Chris and Rihanna -- who won her 7th Gram last night for her "We Found Love" music video -- dropped by Supperclub for a private party ... and according to sources inside the venue, the whole place was filled with pot smoke.

It's the first time the couple's been together on Grammy day since 2009 ... and we all know what happened then.

And there's this -- Chris was shot driving away from the club with Rihanna in the passenger seat at around 3:20 AM ... which would be a big issue if he's driving under the influence of pot -- a big issue because he's on probation for beating his passenger to a pulp.