because your numbers dropped so fast and so dramatically, I would say that even if it WAS twins, both are probably gone :(
You probably haven't passed the actual tissue of the baby yet.
I had a natural m/c at about 5 weeks and I was cramping for days, heavy bleeding, and something the size of palm came out in one piece that looked like a piece of raw liver :S
Sorry, TMI, just trying to give you an idea of what to look for. I'd go back to the Dr. because you may have retained tissue from baby still in there causing the test to still show a positive :(
I had a miscarriage at 16wks last September, and I was induced and had him in hospital but not every part of placenta came out in delivery and 2 days later I had to have a D&C to remove it because it was forcing my cervix open and causing extreme pain.
It's possible you may be getting an infection or something.
So sorry for your loss :(
It could be a twin, it could be an egg.....Not sure. I think it's going to take a doctor's input on this one and some tests. Sorry can't be more help.