HCG levels and ultrasound....WORRIED?

I took a home pregnancy test two weeks ago and it was positive. About a week later, I was having some bad cramping and went to the ER. They did blood work and an ultrasound. They said my pregnancy test was positive, however, my HCG levels were only at 1880 and they couldn't see anything on the ultrasound....no sac, yolk or fetal pole. I had another blood test 4 days later and my levels were up to 6600. They still wanted me to go in for another ultrasound, because they were worried about possible miscarriage or ectopic pregnancy. I am currently waiting for those results. I feel pretty sick, having lots of cramping but no blood. I feel weak and exhausted and I just want to know what is going on. They said it is possible for me to be really early, but that would have meant that i got a positive pregnancy test at 2 weeks pregnant and started feeling sick that early, which isn't common. Anyone have experience with this? Im freaking out!!