Couldn't find heartbeat.

At my doctors appointment on Monday sept 21 I went in for my check up at 13 weeks 1 day and the nurse couldn't find my baby's heartbeat with the Doppler,she brought the doctor in to try and she couldn't find it either. My doctor told me not to worry because I had a tilted uterus and it was hard to find because of it. I had an ultrasound the week before and everything was good. Has this happened to anyone else before? This is my rainbow baby and it has me all nervous and worried.