Pains during after sexual intercourse

I'm officially 18 weeks today with identical twins and so we were celebrating our 3rd year together and for once I was in the mood and now I'm hurting any movement or anything and I was trying to hold out so he made it quick we started very slow and easy but my belly gets in the way a bit. I'm sitting with my legs up a bit and it's very painful in front and in my back. Has anyone had this issue or is just because I'm really small with big babies? If you have tips to help greatly appreciated. I'm also gonna call my obgyn tomorrow.