Does she have any feather loss anywhere else? Ahe may be getting her feathers pulled out by others or herself. Is her lower belly enlarged, or does it just look prominent because of the feather loss? Sometimes you can use Nustock Cream on the skin to help prevent the pecking. It tastes bad to the other chickens, and it has antifungal, antibacterial properties and will help if there are any lice there. Pine Tar will help as well. Both may be found in the horse aisle in feed stores.