Pitbull is making a crazy confession ... he says he was "excited" during a past performance with Jennifer Lopez ... and it's all because she was rubbing on him with her famous booty.

During a Q&A for Billboard Latin Music Week in Miami on Wednesday, Mr. Worldwide recalled his 2011 AMAs performance with J Lo ... and he said she surprised him with a sexy move.

In case you don't remember, J Lo started grinding on Pitbull as they performed "On the Floor" ... and the rapper says that didn't happen in rehearsal ... for good reason.

Pitbull admitted that when J Lo backed it up on stage without any warning, he got a little revved up, so to speak.

It took about 15 years, but Pitbull is finally coming clean ... though lots of folks are saying he's got nothing to be ashamed of.