uncomfortable sex?

My girlfriend and I are both 19. We have a healthy sex life together or at least we did. About 2 months ago she went from taking birth control on the pill, to the shot. Ever since she got the shot the sex isn't really happing. It doesn't matter what we do she claims that It's very uncomfortable and hurts. Could anyone tell me what to do to make it not hurt for her? We haven't had sex in weeks, thank you.