With my daughter I didn't feel her move until I was 20 wks. The day after my 20 wk. appt. actually.
With this one I am almost 15 wks and I feel the baby move, but I really think that it is because I know what it feels like now.
Honestly if you have concerns you should call your Dr. and maybe they can use the doppler to hear the heartbeat or you may even get an ultrasound.
Good Luck!