I also have a low lying placenta and will go back on the 22nd to see if it has moved up. Doctor didn't seem worried at all and said in most cases it moves up on its own as the uterus grows.
i had a previa until my u/s at 26 weeks with this pregnancy, with my other son I also had a previa, but I don't remember when it moved up. They told me it was very common, and actually gave me a higher percentage rate more like in the 90's for it to move and not cause any problems. I wouldn't worry hun, at least not until your much further along.
My placenta was considered low lying and then it moved up at around 22 weeks of pregnancy. Approx 85% of early low lying placentas move up on there own and cause no problems.