Did I lose his twin?

I was reading something interesting about Vanishing Twin Syndrome. I bled so much with my son that I thought I was having a miscarriage. The people at the ER also thought that was the issue. When they finally looked at me they said that my cervix was closed and it had to be implantation bleeding. I got an ultrasound and there was one baby, a yolk sack, and a heartbeat.

After reading about the Vanishing Twin, I have to wonder. If they had done an ultrasound before I bled like crazy, would there have been two babies? I bled for at least a month. It got so bad that I really thought I might have lost my son. I had the cramping and everything. That's why I wonder... do I have predisposition for twins? If so, I really pray there is only one little girl in my womb right now.