I had the same thing at my first u/s (when we first found out about the twins) at 18 weeks. Actually, they were saying it was previa and not just low-lying...not sure if there's a difference there or not. By my next appt, 4 weeks later, the placenta had moved completely out of the way. My OB told me that as the uterus grows, the placenta is kind of pulled up with it.
I wouldn't worry too much about it. Good luck!