When I was about this far along with my son, I was sitting in class taking a final at the university. I started bleeding. I got up, walked to the bathroom, walked to my car, and headed straight to the hospital. I was bleeding pretty heavily and passing clots. The ER doc checked me, and it appeared that my cervix was snug. He said go home and relax. If you are going to miscarry there is nothing you can do. I bled like that for three days. Cried because I knew I was losing my baby. Laid down, kept my feet up......... guess what? He's five now. They never could explain what was happening to me. My doc just reminded me that pregnancy is traumatic for the body sometimes, and it goes through freaky things trying to adjust.
most suchorionic bleeds are not detected by u/s. i can't remember whether it's 2 or 20 per cent of bleeds that are identified. with regard to the hcg tests, personally i would scrap them and stick to ultrasounds as in your case the hcg tests have not indicated what is going on, hereas the u/s showed a viable pregnancy. bleeding with cramps is generally not good, though i had that with this pregnancy and am now 24 weeksand baby is absolutely fine. my bleeding was due to bleeding in and unde the sac which was spotted on u/s because the bleeds were pretty big. i never passed any clots though, and was told these would be a sign i was miscarrying. i'm not sure about just one clot though, things may still be ok. i would also have your progesterone checked as joy suggested.
A SCH wouldn't cause your HCG levels to decrease. Have they checked your progesterone levels at all? If you have low progesterone the uterine lining will thin out and will cause miscarriage.
It could be vanishing twin, too. But I'd get the progesterone checked first.