HCG not quite Doubling

I had HCG lab done on 9/1 and it was at 3029. I went in again today 9/7 to see if they were high enough to schedule and ultrasound. They were at 15,060. I am concerned because that isn't quite doubling every 48 hours. Should of been around 24,000. My Progesterone is at 23.3 which nurse said is promising that everything will be ok but I am still so nervous something is wrong. I have an ultrasound scheduled for Friday. Anyone have similar situation?