Slow HCG rise

I went for my first prenatal appt at 5 weeks 3days, in which I had an ultrasound and all we could see was the sac...my blood was taken to get my HCG levels, which was 2554 and the two days later at 3775. They said the 2554 was pretty low and I need to test again tomorrow bc my levels didn't double as expected! I'm a nervous wreck and wanted to know if anyone had any similar experiences with a positive outcome?