L.S.
Yes, this happened to my daughter and to a friend's daughter not long before. Did your daughter have an extended illness about three months before this? Not necessarily a serious illness - my girl had a nasty flu bug for about 10 days. My friend's child had something else. An extended illness somehow interrupts the hair growth cycle, so you will see a lot of hair falling out about 3-4 months after the illness. It's nothing to worry about. It grows back in.
If she didn't have an illness, I'd check with her doctor. I have no idea what else would cause that, but I bet it's not serious.