A case or series of cases of diarrhea that is so nasty, wet, and/or painful that going to the bathroom feels akin to throwing up from your ass.
Boy: I was sitting on the toilet last night, and all of a sudden everything rushed out at once. It was so traumatizing that it reminded me of throwing up. It was like...
Girl: ...anal vomit?
Boy: YEAH! Man, do I love you.
Girl: That's disgusting. We can't be together anymore.
Boy: What? No! Don't leave me! I NEED YOUR SUPPORT NOW MORE THAN EVER!