What causes a miscarriage?

I am 20 years old, and I was barely 3 weeks pregnant (if even that) and had a miscarriage. I feel too young to have that happen. Does anyone know what causes a miscarriage in a person my age. I have a healthy 3 year old daughter and everything went fine in that pregnancy. I have had 2 abortions, could that maybe have caused something to happen?

I also have Low-grade cervical dysplasia. (IM A MESS). Could that have led to a miscarriage?