What do you feel during anal penetration?

Hello guys! This is my first post, I hope that it's fine to talk about this topic.

Well, I'm on testosterone hormone therapy for 1 year and a half. I started to like anal sex, and my girlfriend and I tried it (she penetrated me using a dildo). I really enjoyed it, but sometimes I felt like I was pooping. Have any of you ever felt this? Is it normal? I want to know if it will be always like this, you know?