I think TJdude is right on track. Sometimes there is a little pain when the penis hits certain spots right in the back. I think it is pretty normal. You may want to get checked by your doc or Planned Parenthood if this pain is really intense or if it happens every time he enters you. Doggy-style is a position that gets very deep and you may want to be careful using that one if you are having pain. Your boyfriend does not have to put his entire penis in you if it hurts, he will still feel plenty of pleasure. Ask him not to go in so far. If he knows it hurts, he will be understanding.

I'm not a girl, but for i've seen some girls in this forum with a similar problem like you, and some of the answers they get from the people here are related to something about the pain being caused by the guy when he hits the cervix during a deep penetration. My advice is that maybe you should id those deep penetration positions where you get the pain and avoid them or try with some positions where you can be on control of how deep he goes, like "women on top". I hope that helps a little, but i bet the girl in here can give you a better answer C ya.