Proctalgia fugax (https://vester.si/bodily-oddities/oddities/proctalgia-fugax/), also known as "shooty arse":

https://www.youtube.com/watch?v=USB90D7ywiE

The video is oddly comforting. "Everyone gets it!"

I remember reading about the phenomenon, that it was colitis (inflammation of the arse, presumably due to gut flora imbalance).