19 weeks no movement

I'm a little concerned. I began feeling tiny little jabs and flutters around 12-13 weeks. It was always when I would lay  in odd positions or strain my abdomen muscles inadvertently. NOW.. It's been about 3 weeks since I've felt anything at all. I don't understand why I would feel him move and then he would just stop. I've had no bleeding or cramping but I'm still worried...