ANAL for the first time?! 🙈

Hey there fellow women! I've got a well established friends-with-benefits situation that I've been involved in for months now, and it's awesome! We're very sexually explorative. Through this relationship I have discovered that I really like various BDSM things, and have learned a lot about myself...WELL, the other day, we did anal for the first time! And I actually didn't mind it!! We were doing it in doggy, and as I like being dominated, he shoved my head down against the couch and stuck a couple fingers in my butt, then removed them, and slowly inserted his wing wang....and it was nice! He said he was shocked by how receptive and non resistant my butt was to entry. Haha! I actually like the feeling of it! HOWEVER - I need tips! I can't shake the feeling during that I'm gonna poop everywhere. During the thrusts, the "out" part feels like I'm gonna poo all over the place, THAT is unpleasant....any way to combat this??? Or is it just a feeling I have to get used to??? I don't think it's poop trying to come out, cuz there's never any on his thing when he removes itÂ