you may have an infection and not know it---a pelvic infection which can causing scaring and make it harder to concieve later on, or it could be endomertiosis. Anytime you have a change like that----all of the sudden it is painful to have sex---then you should go to the Dr.
tell your boy friend to give you time without sex, might be b-cuz you guys have too much sex or he might be doing you too hard, just tell him to be careful,its not about what pleases your boy friend, what matters more is for both of you to feel the same pleasure together. and maybe you can check with a doc to see if you have an in fection.