Reasons can hurt please...?

Now I am 24 and have a very healthy drive. I have been with my boyfriend two years. But have always kinda had this problem. I can have missionary and feel fine, its the moves like from behind that cause me mild pain. I thought maybe it was my boyfriends size. When we first started sleeping together it took a little bit because he is on the larger size. But that went away. Then I thought it was because we needed more lube. And that isnt the case either. I have gone to my OBGYN and they tell me to use more lube. Well thats not the problem. The pain is around where you get cramps. It is a sharp achy pain. And sometimes it will hurt for awhile after we are done. And I know he must be hitting my bladder because I always feel like I have to pee when we are in that position. I know you are gonna say well dont use that position. But when we get in this one position it feels wonderful. And when we have its just not about pleasing me I want to make it feel good for him too. And he knows its a problem. But unless it hurts really bad I just bare it. Because my boyfriend will take it personal and he thinks that everything hurts. He is so sweet tee hee. So I wondered if anyone else had this problem and what it is. I know I should go back to my doctor. And I will but in the mean time. Can you help? Thanks.