Britney Spears Drops F-Bomb on Rude Fan (VIDEO)

10:45 AM PT -- It now sounds like the guy who said "fat bitch" is so far back there's no way Britney could have heard him. So the question ... what happened that pissed her off so much? Several sources who work on the show tell TMZ, they believe Britney was reacting to a comment someone made in the audience, but they don't know what was said.

Check out the video closely. It looks like the music may have started late ... so that's a possible trigger as well. But no one is saying definitively.

Britney Spears stood up for herself Wednesday night during her show, when someone in the audience apparently called her a "fat bitch."

A guy standing next to the mic says "fat bitch", and you can clearly hear Britney say ... "f***ing asshole."

It's interesting on several levels. Critics of her show say she walks through her performance like a robot, but this clip clearly shows Britney's antennas are up.

Plus, her mic was on ... during a song!!!

And she doesn't take crap from random people. Are you listening Sam Lutfi?