Slow fetal heartbeat

I went to the doctor today for my first ultrasound. I didn’t see any movement so that immediately alerted me since I was supposed to be 8weeks today. After they measured the embryo and tried to get a heartbeat, I started to panic because I couldn’t see anything. Afterward I saw the doctor and she changed my due date making me now 6weeks instead of 8 and the heart rate was only at 77bpm. I’m trying to stay optimistic for the next ultrasound next week but I’m really scared. Any advice/ has this happened to anyone else and they had a full term baby? This is my husband and I’s first pregnancy so we don’t know what to expect.