Is she with a rooster? I once had a silkie pullets head stripped like that from an overzealous roo. It doesn't grow back too quickly, but should when she moults. She could also just be in a moult. Examine her vent area for mite/lice sacks to rule that out. Typically a light case of mites would not cause this, but an infestation may and sacks would be noticeable.