No heartbeat..

I had an ultrasound at 7 weeks 1 day and saw the baby and heard the heart beat, it was 115. Everything was perfectly fine. Today they checked for the heartbeat with the doppler and couldn't hear it. My doctor said that my uterus is tilted back and everything measures fine and not to worry. But I can't help but worry. My next appointment isn't for 3 weeks. I just need reassurance. Any similar experiences and everything was fine?