Could be she rubbing it on something and the hair is coming off. Another thing is an allergic reaction to something that is on her face.
Is she scratching a lot? Have you introduced anything new to her cage?
Some mites are microscopic and you can only see them with a skin scraping. This seems to have come on very quickly. If there's nothing new in her cage (food, housing or bedding or cleaning solution), keep an eye on her, see if it continues to spread. She'll have to see the vet then.
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