Pain during sex??

Mariah
Me and my SO were trying to have sex last night and he got about 10 thrusts in and I kept trying to pull away and then I was like babe I can't it's hurting and it wasn't hurting me too bad but it was enough to stop and he was feeling so bad. But it was feeling like he was just hitting the bottom of my insides like as if I wasn't pregnant and we were going at it but we weren't going at it hard or rough we were just getting started? Anybody know why it was hurting me like that?