I agree with the other poster.
I'm also wondering if you could have a vanishing twin...that would cause your hCG levels to temporarily drop while the sac for the remaining twin grew. so...hang tight and ask for further testing in a week...both blood work (hCG) and an ultrasound.
If your numbers are dropping it is not a good sign.. If the sack is still there you havent passed the tissue but could be in the process of miscarrying now... The bleeding could come back and evenually turn heavier... You test can continue to show positive for a bit after you pass the tissue as well.. After mine, I was still testing positive two weeks later but my HCg was alot higher at the time so it took a while to drop.
Given the fact that your numbers have dropped I would say you are probably going to miscarry.. Its interesting that the sack continued to grow, If you dont start passing anything in a weeks time, I would get a new U/S done. Some woman dont end up naturally passing it and may require a d&c.. but before you go that route, give it some time and get check out again.. Because stranger things have happen.