Like the last poster said it sounds like a clot to me. They are pretty common in early pregnancy where there is a healthy baby but there was spotting and cramping. Your levels look fine to me since the normal range for HCG levels is this (or so says the American Pregnancy Association)
3 weeks LMP: 5 - 50 mIU/ml
4 weeks LMP: 5 - 426 mIU/ml
5 weeks LMP: 18 - 7,340 mIU/ml
6 weeks LMP: 1,080 - 56,500 mIU/ml
7 - 8 weeks LMP: 7, 650 - 229,000 mIU/ml
9 - 12 weeks LMP: 25,700 - 288,000 mIU/ml
13 - 16 weeks LMP: 13,300 - 254,000 mIU/ml
17 - 24 weeks LMP: 4,060 - 165,400 mIU/ml
25 - 40 weeks LMP: 3,640 - 117,000 mIU/ml
For me I had levels about as high as yours if not higher at 8 weeks and I now have an VERY healthy baby.
i know when i had a molar pregnancy, there was nothing even in my uterus at all, no sac or anything..maybe you just passed an old clot of blood or something. i really dont know. but YAY for the baby that is in there!!!! :)