if you turly are 7 weeks then you would have seen something. Your numbers sound like you might be about 5 weeks along maybe? You won't see much of anything even at 6 weeks. Try to relax, you can't tell if anything is wrong with out seeing how your numbers are doing (and even then it's hard to be 100% sure).
If they didn't see anything, you may not be far enough along yet. How long ago did you have the first u/s. Doctors will usually wait a week in between b/c that will show the most progress. Also, your hcg won't mean much unless you get a second blood draw about 48-72 hours later to see if they are rising. Good luck, I wouldn't worry too much at this point. You only have half of the puzzle so far. =)