Anal for the first time!!!

So I’m trying to switch things up with my bf. I’ve been thinking about maybe trying anal for the first time. I’m a little worried because once he fingered me and I felt like pooping right after (TMI lol). Also I’m insecure because I suffer from constipation and I feel like once he takes his penis out poop will come right after hahaha. Can I have any useful advise? How was your first experience?