U/s, so terrified, please help

Since they were worried about the heart rate, I had to go in for a second ultrasound. They insisted I was 6 weeks because of the size and my LMP but I know for a fact I ovulate on day 18 and I have 31 day cycles so according to my calculations it was more like 5 weeks 3-4 days. Anyway, the hb was 92 the previous day and 94 the next. Doc said 50% chance of miscarriage and gave me a progesterone injection. A midwife I know told me that sounds ridiculous and heart rates in the 90s are completely normal in the 5th week. What do you guys think? I'm terrified and very worried. 

HCG went from 7000 to 14,500 in around two days.