Nikki Harbor, the black woman who recorded the wild confrontation, tells TMZ "Karen" -- real name Erin -- was already arguing with her husband when she and her boyfriend Isaac pulled up to the gas station ... so she started to record.

Nikki tells us when she started to film the argument, Erin noticed she was being recorded and approached Nikki and Isaac's vehicle. Nikki stopped recording and claims Erin then pointed at Isaac, who was pumping gas, and called him a drug dealer.

Nikki assures us, she and Isaac had never met Erin before, so the idea she knows anything about either of them is preposterous.

That's when the now-viral video picks up, where Karen's going off on just about everyone ... while her husband apologizes for her rude behavior and tries to get her to leave with him.

Nikki says the entire incident was clear racial profiling ... and Erin had no reason to ever insinuate, much less accuse, Isaac of anything.