Why do I get pain during sex?

I am a small girl, I'm 5 foot 2 inches and 100 pounds, and sometimes I get pain during sex in my pelvic area. It's a sharp pain and doesn't happen every time but can happen in almost every position we've tried. I keep reading things about terrible diseases that could be causing this like PID but I don't get pain all the time and it's really not that bad when I do so I do not think it's that. How can I stop this pain and why is it happening?