It sounds to me like she is molting, which is common at this time of year. When birds molt, they lose feathers all over their body (but usually first on their back, neck, and tail area), stop laying, develop pale, shriveled combs, and may act somewhat abnormal (more lethargic, skittish, etc.). Chickens don't always molt at the same time as eachother, so even if your other birds aren't molting, she still may be. To help her with her molt, try giving her (and the rest of the flock) high protein treats, like scrambled/hardboiled eggs, mealworms, cat food, or other insects.
However, there could be something wrong with her. I'd look on her skin/feathers for moving black specks (mites), yellowish moving blobs (lice), or white eggs (lice eggs), as external parasites can cause feather loss and lethargy. If she has mites/lice, treat by dusting her with Sevin dust, spraying her with a lice/mite spray, or dusting her with poultry dust. Dust again 7-10 days after the first dusting, and make sure to dust/spray the coop, too. Also, has she ever been wormed? And how old is she? While not related to her feather loss, a heavy burden of worms could cause her lethargy and paleness. Other things to check are her crop (is it unusually large/hard?), her body (is her abdomen hard and/or swollen?), eyes, face, and nostrils (is anything swollen or is discharge present?).
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