Missionary position really hurts my penis. Does this happen to some people?

Whenever I try missionary position with my girlfriend, my penis seems to bend a lot inside of her. It is rather painful and feels as if it is bending too much and might snap if I push it too hard. I have not noticed this problem from the side, or doggy, or with her on top. My penis doesn't seem to be under as much pressure to bend in other positions. Why is this? Am I alone in this issue? Is there anything I can do to make missionary less painful.