Anal aftermath

So I’ve come to eve because I’ve always wondered and cannot find an answer anywhere on the internet.

Every single time my partner finishes inside my butt when we do anal the next day I have runny poos all day. Like not rlly bad like watery. Like I legit have been toilet today 3 times and if I was to fart I would shit in my pants basically. Haha this is such a weird question I’m just so curious.

Is there a reason this happens does his sperm do something. It’s not annoying or painful just random.