HCG Levels?

Be
So I've done a blood test every Tuesday for the last 3 weeks for my hcg levels. 
First- 2725
Second - 31,948
Third - 126,000
Today I had my first ultrasound to determine how far along I am and they put me at 6 weeks 2 days. I'm curious because my levels are way high for only being 6weeks and everyone else's levels seem to be much lower. Ultrasound showed one small  baby. Anyone have any insights on this? Thanks!