Two possibilities. One is that the sac was there before and just not apparent to the sonographer, and you had a miscarriage. The other is that twins began, and the first time you were in for an ultrasound the sacs were too small to appear on the u/s. Then one faded (causing the drop in hCG levels) and the other continued to develop, and that one can be seen now on the ultrasound. Get another hCG draw in two days, and if the numbers are still dropping, it is definitely a miscarriage without the vanishing-twin stuff.
Usually your levels are suppose 2 double every 48hrs... Do u kno how far u are?