Third time having sex!

Tonight was my third time having sex, and we are still having to do missionary. It doesn't hurt at all other than the initial discomfort of the first stroke that goes away in about a minute. Because it wasn't hurting at all this third time, we wanted to try me on top and doggy style, but when he turned me over, the pain was awful so we just went back to missionary. Is this normal and how long does it normally take to get in the rhythm of sex to where you can try all new positions?