A UFC fighter got the literal crap beat out of her Sunday night -- and left the proof INSIDE THE OCTAGON!!

The fighter is Justine Kish -- who was trying to wriggle her way out of a killer rear naked choke courtesy of Felice Herrig during "UFC Fight Night" in Oklahoma last night ... when her bowels betrayed her. 

Kish lost the fight ... and the contents of her colon.

Afterward, Kish joked about the situation -- saying, "I am a warrior, and I will never quit #ShitHappens haha be back soon."