NFL's Cameron Jordan -- Alleged Ass Slap Triggers Investigation ... Report Says

New Orleans Saints defensive lineman Cameron Jordan is reportedly under investigation for slapping a woman's ass at a New Orleans nightclub ... and then fighting with the woman's BF.

The incident all went down at Republic nightclub this weekend, according to WWLTV in New Orleans ... which is reporting the New Orleans Police Department is already on the case.

According to the report, Jodan allegedly had some words with the female accuser and then slapped her butt. Her boyfriend reportedly confronted Jordan in the bathroom and a "minor scuffle" ensued.

The couple filed a police report ... which also says Jordan sprayed them with champagne inside the club.