HcG riding slow & empty sac. Positive stories?

Jen • Mom to one very cute little boy conceived with the help of Glow! Avoiding baby #2 for a few years
I went to my first U/S at what i thought was 7w but dr only saw an empty 8mm sac. I had HcG done and it was 3351 and rose to 4007. Although the dr didn't wait 48 hrs I had it done at 40hrs. But he said he was concerned it didn't double and wants me in for another U/s on fri. I am trying to stay positive because I know dr is not considering I have a 32-34 day cycle I got a OPK+ on cd19 and implantation cramping on CD28-30. Which would mean baby has only been doing it's thing for about 3-4wks. Anyone have similar stories? Any hope?