Belly dropped at 24 weeks.. Worried

Hi ladies ,I'm 23 weeks 4 days pregnant and my belly dropped low today and I'm really worried that it might start labour..I'm seeing my doctor tomorrow and she said not to worry over the phone as I don't have any other symptoms..but how could I not worry ,that's all I can think about..I really I can carry to full term as I didn't even gain enough weight yet for the baby to be healthy..