Pain during sex

I've been sexually active with my boyfriend since February, and he was my first. It hurt the very first time but it was expected and not very bad, since then i haven't had any problems regarding pain during sex that i couldn't explain or pin point the cause of. but this last time, it hurt quite a bit when we first began, and so much more after it was over, it was like having period cramps inside my vagina, a sensation I've never experienced before. i hope it's just a strange day in my cycle or a temporary side effect of my new birth control, but i was wondering if anyone else has experienced the same thing or something similar.