what you have described sounds perfectly normal to me. he is hitting your cervix. the positions you describe (doggy, legs pressed towards chest) are the same positions that give me pain during sex. positions where the guy enters you from behind can hurt, and so can missionary when you press your legs up against your body like that because your vagina is shortened in this position.
your cervix changes position when you are aroused and at certain times of the month - perhaps you are not sufficiently aroused before having sex. i hope this helps:
"After menstruation and directly under the influence of estrogen, the cervix undergoes a series of changes in position and texture. During most of the menstrual cycle, the cervix remains firm, like the tip of the nose, and is positioned low and closed. However, as a woman approaches ovulation, the cervix becomes soft and mushy, and rises and opens in response to the high levels of estrogen present at ovulation. This change, accompanied by the production of fertile-quality cervical mucus, supports the survival and movement of sperm. " - wikipedia