Need some advice please!!

Hi, I'm 20 weeks pregnant and really worried I thought my waters may have gone the other night while me and the hubby was doing the deed but midwife said due to no leaking on pads its unlikely but I'm bit worried as my belly has gone from rock solid to barely there and really soft, please tell me someone else has gone through this.

This is my 4th pregnancy 3rd baby but my waters went early on both my other pregnancies and I knew it was my waters as was in hospital at the time.