Low fetal heart rate.

Rubi
A week ago I went to the ER for a nasty stomach bug, I was then 7 weeks almost 8 according to my last period. The doctor recommended we have blood tests and an ultrasound done to make sure everything with baby was ok.
My fetal HR was in the 50s!! Two days later I went in to have a check up and baby grew and HR was in the 90s which was still low for 8 weeks.. i have my next appt tomorrow but was advised to be aware that I can miscarry because baby will not survive if the HR doesn't increase.. I'm super anxious and prepared for the worst. But has anyone else experienced this? 
I know my time might be off and I may not be as further along as I thought, but this is tough..