Pain
I've been with my boyfriend for 5 years and generally we have few problems when it comes to sex. One thing that has come up however is that when we have sex in doggy style i feel a strange pain in my lower left abdomen (just to the right of my left hip bone). I know that this can be avoided by avoiding the position but I was wondering if there was any significant reason why this could happen. I've never felt it in any other position but I have had pains there that seem to spring up from nowhere.
Doggie style is a position that a lot of people love because he can get pretty far inside of you and he tends to hit the g-spot this way. However, it can cause a bit of pain if he's a big larger (or longer), or if you don't use enough lube, or if he tends to "jackhammer" in this position.
If you haven't been tested of STIs recently, you might go ahead and do that if you think it might be your cervix that is hurting, as this can be a sign of an STI. Otherwise, know that this position can hurt some women regardless!
Hugnkiss, I would add that for all the people who enjoy doggie-style sex, there are plenty of people who don't enjoy it because of that very reason you're describing. If you want to continue trying sex in this position, it is important for you - the receptive partner - to try to control the pace and depth of penetration as best as possible. Hopefully that will alleviate some of the discomfort.