I had a very low lying placenta as well at the 20wk scan. I was scared as well when they mentioned the c word! Mine moved up out of the way and in most cases it does. I had my repeat scan at 33weeks and all is clear.

Feeb's is right it is quite common. Try and enjoy your pregnancy and just think you will get to have that extra scan at around 34weeks to check in on your little bundle!