sometimes when i am having sex with my bf in a missionary position, i get an intense pain that feels like my upper vaginal wall is caving in. i can't move or turn over or anything, and if he's still inside of me, him moving causes even more excruciating pain. it's so bad that i cry till it goes away (sometimes an hour) i've seen my dr about it, but she says everything looks fine. any idea what can be wrong?