EXCLUSIVE

Porsha Williams attacked a woman during a recent 'Real Housewives of Atlanta' shoot because the chick brought the wrong dude to a holiday party.

We're told cameras were rolling when Porsha's good friend Jami Ziegler floated into the shindig on the arm of a man, and something about that man pushed Porsha over the edge.

Sources say Porsha first tried to get Jami booted from the party, but she left on her own. We're told Porsha followed Jami outside, shoved her to the ground ... then started punching and pulling hair.

The fight was broken up quickly enough that cops weren't called, and there were no serious injuries. We're told cameras even missed the action, so ... someone's getting fired.

As for what set off Porsha? Word is she's got bad blood with Jami's guy, though it's unclear why. We reached out to Porsha, no word back yet. But warning to 'RHOA' cast -- someone's still got quick hands.