Correct me if I'm wrong, but isn't that her breast and abdomen in the pics? If she is molting, you would see feather loss in more parts of her body. I would dust her with poultry dust for mites. I would also watch to see if she is spending extra time in the nest box. Some hens that are building a nest to start to brood pull breast feathers out to line the nest.
This was my thought too, that she is broody.