Belly size at 19 weeks??????

Hello ladies - okay this is my first and some days my belly seems to be popped out and today it seems small. I am not in maternity pants yet - just wearing my old pants with a saftey pin. Is this bad, it makes me nervous. Does the belly fluctuate depending on water retention, bowels and where the baby is??? I had a lot of round ligament pain last week but this week I feel really good. Last week I went to the dr. and the heartbeat was at 160 i am jsut nervous this week b/c my belly hasn't popped and I don't feel it moving or round ligament pain.