Help! My HCG isn't doubling!

I am extremely nervous. When I went for our first sonogram on Tuesday, I was supposed to be 8 weeks. Unfortunately, I was only showing I am about 4 weeks. My period is normally irregular so we were hoping that I just ovulated late. I got blood work done that day (Tuesday) and I was at 991 HCG levels. I had another test done on Thursday and only had a 1493 HCG level. My doctor says I might be on my way to having a miscarriage, or the baby may be in my Fallopian tubes (it's too early to tell she said) or she said there is a small chance things might be ok. Is there anyone else out there that has been through this? I'm panicking.