Butt pain during sex

HELP!

I saw a post here but I didn’t really have much people who could relate or help.

When I have vaginal intercourse with my boyfriend and if I’m on top of if he thrusts everything in, I get a sharp pain in my bum. If he doesn’t put everything in, only half or almost everything, it doesn’t hurt.

Is he hitting my cervix and that’s why I get this pain? or is it just because he’s on the bigger side, or is something wrong with me. Anyone else experience this. Any tips???