Could it be lice or mites causing irritation? Is another bird plucking her and causing an infection? Is she getting enough protein? Is she being over-bred by a rooster?
Try washing it off and putting ointment on it and see if it gets better. Could is possibly be a mild burn?
The depluming itch mite is another quite small (~0.5 mm) that infests domestic birds such as chicken, pheasant, pigeon and goose. However, it is not as frequent as other mite species and prefers warmer climates. It does not suck blood, but burrows into the skin along the shafts of the feathers, which easily break. Affected birds bite themselves vigorously and may pull out their own feathers. It can also cause cannibalism among the birds. The lifecycle can be completed in about one month. Transmission is by direct contact between the birds. Infestations in modern operations are rare, more frequent on free ranging and backyard birds. For diagnosis a a few feathers close to a lesion can be pulled out and examined under the microscope. http://parasitipedia.net/index.php?option=com_content&view=article&id=2540&Itemid=2816
Get yourself some of the product to treat poultry directly and on premises as mentioned on that site.