Hello and Welcome to BB.

Your numbers are looking good. They do double in the early days but then they start to slow down so try not to stress to much.

As far as your dates go, do you chart? Maybe you ovulated later than you think and therefore you wouldn't be as far along as suspected or if you use your LMP date.
My cycles are 48 days and I do not ovulate until about day 28 so my dates would be 2 weeks off if I used my LMP date instead of when I ovulated, do you know what I mean?

How did the ultrasound go, 6 weeks is still early but were they able to see a HB?

I know that it is hard not to stress... but really the HCG looks good and dates can vary slightly.

Wishing you a happy and healthy 9 months.