PIV feels weird during doggy style please help me fix it?

Hi. I don't know if this is the right subreddit but. Whenever I have PIV sex with my partner coming in from the back, my butt feels weird. Almost like he's thrusting up into my butt? I hate the feeling because it lowkey makes me feel like I need to poop. I do love positions like doggy so I don't know how to fix this. Not sure if this is relevant but my boyfriend is average-ish long but very girthy & his head bends up/sideways a tad bit. What could be the problem and can I train myself to stop feeling this way because this is relatively new, sex from behind didn't always feel this way. Thanks for the advice in advance!