If your really concerned you should go to the doctor right away , your baby could be dead. my mom had the same thing happen and she lost her baby.
if your concerned i woud speak to your doctor now just to be on the safe side it could mean nothing but just edge on the side of caution its best to know then worry yourself when everything could be fine your still only 16 weeks still little early to feel any big movements just call doctors make appointment to stop you stressing goodluck