not to discount your gut feeling, but i was so absolutely certain i had miscarried because i spotted/clotted/bled for 9 weeks earlier in my pregnancy (i'm now 21 weeks along with a perfectly healthy baby and no sign of fetal remnants at any point to suggest a missing twin). the spotting started at 6 weeks, increasing to clotting at 9 weeks, and became bleeding at 10 weeks. at 10 weeks some of my pregnancy symptoms totally disappeared and because i have experienced several miscarriages in the past i freaked out and call my midwife. my midwife got me in right away to do an ultrasound and showed me that, in fact, my baby was alive and kicking! i had what's called a subchorionic bleed, and are actually not all that rare in early pregnancy. sometimes it's severe enough to warrant bedrest (which happened to me with both my living children), and others pose no risk at all. what you're describing sounds a lot like the bleed i had this time around. my bleed was finally considered gone at 14 weeks, and like i said, we're having a healthy (and already chunky!) baby girl in early july.
that said, your doctor should have done an ultrasound to rule out vanishing twin syndrome. i would discuss your concerns with your doctor at your next visit, and see if he can't order an ultrasound now. however, it would likely be inconclusive, as typically in vanishing twin syndrome the dead fetus is reabsorbed into the placenta and even by the remaining baby and rarely passed by the mother when it occurs in the first trimester.
good luck to you!