I, just like you worried about that as well with my first pregnancy. My ds was all over the place and seemingly constantly on the move. Every prenatal appointment I had I asked my O/B if there was something wrong. He assured me that "an active babe is a healthy babe!"
He said that at every appointment!! By the last month the baby is so crammed in there and very uncomfortable that they spend a lot more time trying to find a comfortable position. I wouldn't worry one bit!! Like Valerie said, worry when there is no movement!!