Worried about Heartbeat.

I had my first ultrasound yesterday, and for the first time ever I was able to see my baby, see the heart pumping away. I broke down in tears I've had 2 MC and I'd never seen my little once before. I was over the moon, my SO and I couldn't be happier but my doctor called that evening. I just called her back today. My baby's heartbeat is only 81 at this point (8weeks) it should be 120 or higher. Has anyone had something like this? I'm so scared that something is going to go wrong, I'm shaking. I don't want to lose a third baby, not now, not when I finely got to see him/her.