Could she be molting? How old is she? Are you seeing more feathers than usual lying around? https://www.backyardchickens.com/a/chickens-loosing-feathers-managing-your-flocks-molt and just in case: http://anrcatalog.ucdavis.edu/pdf/8162.pdf
How old is she? It looks like she's molting, which is common in hens at this time of year. If molting is the case, you should start seeing feathers growing in within a few weeks. If not molting, it looks like damage caused by a rooster mating her or other birds pecking her. I would observe your flock for an hour or so and see if anybody is picking on this hen too much.