hard to find slow heartbeat

Erinn
My doctor called because my thyroid wasn't working and put me on medication. I  went in for a ultrasound and the heart beat was hard to find and slow.. they sent me home and told me to come back next week. I'm very nervous. I've had a miscarriage before because they couldn't find a heart beat at 9 weeks. I'm 8 weeks now. Has anyone else had this happen??