Fx for you, Allensmom.
Okay, I have a couple comments...
First, HCG levels vary a lot depending on the person. My doctor was worried I had an ectopic since my HCG was 15,000 at 5 and a half weeks, but it turned out fine (I know I O'd right on time but got a positive test only 8 days after, so I think my levels just rise quickly). A high HCG could indicate you implanted early, or your levels just tend to rise quickly.
Second, having a high HCG does not indicate what they will find on your ultrasound! It's true that there are certain "lower thresholds" of HCG. If you are below those thresholds, it really won't be possible for them to see certain anatomy in an ultrasound. However, there is no "upper threshold" for HCG, meaning that you could have a really high HCG and still be seeing less on the ultrasound. Certain structures take certain amounts of time to grow, so if you're (for example) 5 weeks you won't see the same structures as you would at 7 weeks even if your HCG is as high as some women are at 7 weeks seeing those more advanced structures.
Third, how sure are you on your dates? Is it possible that you may have O'd early? Sometimes women can O early, which could throw off your dates a bit.
Fourth, ultrasound scans can be off. Most doctors say that they have a margin of error of about 5-7 days. So that "6 week" sac might actually be 5 weeks, and you may have ovulated early so are actually at 5 weeks now. It's not an exact science.
I hope you're able to get some answers soon!