decreasing HCG?

I found out I was pregnant extremely early, like within days of having sex. And shortly after I had a positive pregnancy test I started cramping. I went to the ED and had an ultrasound but the pregnancy was so early they couldn't see anything. So we've been following my HCG levels for the past 2 weeks and planning on having an ultrasound next week. Well from last Thursday to sunday, my HCG level went down instead of doubling like it's supposed to. Has anyone ever experienced this and had a positive outcome?! I'm so anxious and stressed out. Praying I don't have a miscarriage