Hcg levels

Allyson
So after an ultrasound with no heart beat (there is a chance it was too early) last Thursday, i had blood work done the next day and had an hcg level of 37,000. 
Comparing that to the following Monday (where it should have doubled) it went up to only 39,000. 
Has anyone had experience with this?
I'm getting my blood drawn again tomorrow morning (Wednesday) and a second ultrasound Thursday afternoon.