MC or vanishing twin syndrome?💔

Long story short I tested positive for being pregnant with a blood test. It was 136.2 it only went to 180 in 2 days. Then the next day (june 4th) it dropped to 160 and i passed a tissue clot that was grey and a blood clot. I rushed to the ER and they said it was a MC. Two days later I went to my ob to make sure my hcg was dropping but it jumped to 320. Then 496 and now it's 900 as of friday. I got a ultrasound and they seen nothing there. But my symptoms are very strong. I go get a fetal MRI tomorrow and I'm nervous. Did I lose one baby and keep the other? They checked my tubes and there was nothing there and there was no left over tissue.. I'm so confused and stressed. I tried to explain to the ob that my cycle is 37 days and i don't ovulate every 14 days.