Nothing visible on ultrasound, no mc symptoms

I think I'm about 6-8 weeks along and have had 2 ultrasounds. Both showed nothing in my uterus and nothing in the Fallopian tubes. My (temp) doctor thinks it's abnormal and I'm about to have a miscarriage, but I have no miscarriage symptoms (just minor spotting). My hcg level was a little over 8400 a week ago (on 3/18) and had another blood draw today. What could be happening?