Caitlyn Jenner: That's Not My Boyfriend ... That's My Kid!!

Caitlyn Jenner and son Burt got mistaken for boyfriend and girlfriend ... hilarious for sure, but it soon turned ugly.

Caitlyn and 37-year-old Burt Jenner went to lunch Monday in West L.A. when photogs jumped to the conclusion they were hooking up and peppered them with questions.

Check out the video. You see Caitlyn leave and Burt blocked one of the photogs so he couldn't follow her. When the photog gets out of his car -- presumably to confront Burt -- Caitlyn's son goes on the attack.