HCG levels not doubling

My hcg numbers on Thursday were at 24,197 on Saturday they went only to 25, 798 I should be about 6 weeks according to this app but I'm thinking maybe only 4. I had an ultrasound Thursday and Doctor only found a sac no baby. I'm really scared I'm going to miscarry. Any ladies going through the same or have any success stories. I'm losing sleep and so very sad.