I only saw a sac at 5 weeks (from my last period) and at 7 weeks I saw my baby happy and with a heartbeat.
The way doctors count pregnancy since the date of your last period is confusing because you might have ovulated later than you think. If the actual conception occured later the baby would be too small to be seen so early on.
Your HCG level is doubling, so try not to worry, repet the ultrasound and remember that there could be other explanations, such as the one above.
I hope it's not a blighted ovum, I was also worried about that before I actually saw the beanie on the monitor. My husband said I should have restricted internet access