HCG levels question - please help!

El
So I had to go to ER on Saturday because I was bleeding. At the time I would have been 6w1d pregnant. My HCG levels were 26,760 . They could see a sac only no baby or heartbeat.  
I had blood work taken Monday 6w3d and my levels were 31,502 ... my dr says this is normal but I thought the levels had to double? 
I appreciate the help!!