Anal question

So my S/O and i have been having sex for a while, but recently all he talks about is anal. He really wants to do it but im a little nervous. We attemped it before but i freaked out cause it feels like im about to poop the whole time lmao. It didnt hurt but i cant get the thought out of my head "what if i shit" is that even possible?? Lmaoo educate me girls please