She really needs to be seen by a doctor. There are a lot of reasons and some of them preventable. I'm no expert, but a doctor needs to investigate with someone so young.
There is a ton of information on the Internet and a lot of bad information too.
Here is an article which lists many possible reasons. Getting to the Root of Female Hiar Loss, from MSNBC. There are tests which need done apparently.
Or you may see something in that list which rings a bell. Like medication she is taking, or some other reason may stand out. Good luck.
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