Painful sex issues?

Kate • Just a Psychology student tracking her menstrual cycle
My boyfriend and I are fairly... Active in our sex life. Though the sex is pretty vanilla, it occurs really frequently. If not every day, every other day. We're young (early 20's). We like to have fun. I've only recently been *so* sexually active, and I've noticed that a lot of the time after sex I am sore. He isn't rough with me. And after I told him I was sore, he's become even MORE gentle. I do have a really small vagina, but he's fairly average sized. I just was wondering if any ladies out there could give tips on how to help or maybe reasons why it's sore? (: thanks!