Stressed!!! Where is my baby hiding???

Maybe I'm getting worried for no reason, but my Dr can't find the heartbeat and neither can I. I'm 18 weeks now and I've been able to find the heartbeat on my own since 11 weeks. All of the sudden can't find it, and to top it off I'm losing weight and feel so much smaller. Not really feeling any movement either.....am I being paranoid or is it something I should call my Dr about???