Even if you were having twins your number would still be rising even if one was lost. Sounds like you are having a posible miscarriage. You need to talk to you doctor asap!
I agree with tarajanes, the numbers should be doubling or continue to go up even if you lost a twin. Have you had an u/s? What has your DR said? Are you having any bleeding or cramping? The best thing is to talk those numbers over with your DR. Plz keep us informed on whats going on.
Its possible but it sounds like you maybe miscarrying.you numbers shouldnt be dropping but doubleing,even if it were a twin your levels wouldnt be dropping that much.