Why do i feel pain in my stomach when i have sex?

Ok so last night my bf and I made love for the 1st time and I had discomfort in my stomach. I'm not a virgin but I only had sex 3x b4 him and they have been spread out months && yrs apart. I was thinking that it was because he was a lil big since he had trouble getting it in and then it hurted a lil bit after he finally did enter me. I was also thinking that maybe it was because I was holding my gas n the entire night. Anyways it don't hurt to the point where as though we have to stop completely but sometimes we have to stop that position for awhile , like if I sit straight up on top of him && bounce it hurts when I come dwn vs. When I just rock back and forth or if I arch ova top of him. And if we are in missionary position it hurts if I open my legs up too widely or if he have both legs up n the air. He's not rough with me he takes it gentle so it can't be that he's hittin it too hard but what else could it be ?? And is this gonna last forever ?