6 weeks trouble with heart beat

I'm a little over 6 weeks pregnant and had an ultrasound. We could see the heartbeat flickering but my doctor had a hard time hearing it, it would come and go get but we could see the flicker the whole time.  I have to go to a high risk doctor to get a better look. My doctor said it could be nothing but it could also be a problem. I'm so scared, I already had a miscarriage before this pregnancy. Anyone go through anything like this or have any insight?