Holy smokes, this is early! You must be cranking out the HCG. Congratulations!
It's actually about right for a singleton from my experience. Twins tested positive 4 days after transfer
When I became pregnant with my second the nurse asked me if twins run in the family because my numbers were pretty high. I nearly fainted. I think it was due to me having a rogue egg at the end of my period, so the baby's measurements were always off from my LMP.