I've got a low lying placenta too - 2cm from the os. 90% or so of low lying placentas picked up at twenty weeks have moved up considerably by 32 weeks or so. The risk factors for this not happening are previous c-sections, advanced maternal age (that's me, 39), and a high number of previous pregnancies (that's me too). I'm sanguine about the higher risk that mine won't move up but this happened with #4 too and it moved up and I'm not particularly anxious.

I'll be having a quick follow up scan at 30 weeks to see how things are going.

And yes, a posterior placenta means you can feel lots and lots of movement! This baby never seems to stop wriggling!