Kind of freaked out

So I'm 5.5 weeks. I had some mild cramping a week ago and they did blood work and scheduled an early ultrasound to make sure it wasn't ectopic. My numbers (hcg and progesterone) were where they should be. During the scan the measurements were good and we saw the heartbeat. However when I talk to the dr she is very concerned that it was only 78 bmp which is very low as they want it over 100. I now have to wait 9 days to a follow up to make sure the heart is beating faster. I know there is a possibility that it just started beating or it cod be a sign of an impending miscarriage. I don't know how I am going to get through 9 days. I don't really know if it's advice I am looking for it just needed to say that outloud.  Thank you.