Why is your 12 year old daughter bleeding?

If she is bleeding from the vaginal opening then it is because she has her period. This is when you bleed once a month for about 5 days. The process of having your period or menstruating is getting rid of the liner of your uterus because you are not pregnant. Every month a new liner is put in place and if you don't get pregnant then it will "empty" this is when you bleed. If you do get pregnant then you usually miss a period which is because your womanly insides are preparing to carry a child.