I've heard and been told by the doctor that at this stage (until we hit 26-28 weeks) it's normal to feel the baby move a lot and then not at all. It's not that your baby isn't busy in there its just that it's not strong enough to be felt all the time. If you're really worried though I would schedule an emergency appt. They are there to put your mind at ease. Just try a rend stay calm and understand that it's perfectly normal. The same thing happens to me at ended I'm 19 weeks as well.
You can buy a machine at amazon or maybe even Walgreen's for around $50 to hear the heart beat whenever you want.
Try communicating with your baby. Play some music. My baby love classical music and guitars .
have something cold like iced tea with honey and see if baby moves?