HCG levels?

So on 12/16 I went into the ER and they diagnosed me with a threatened miscarriage, my hCG levels were at 195 (I was 5.5 weeks). I stopped bleeding on 12/17 and had my levels retested on 12/21 and saw my doctor the same day she confirmed a miscarriage as she didn’t see anything on the ultrasound but my hCG levels increased to 851 instead of going down like I thought it would. She wants me to retest again and get another ultrasound done next week. Has this happened to anyone?