I agree that it sounds like she may have a respiratory disease, such as mycoplasma (MG,CRD) or coryza. Those usually requires antibiotics, such as Tylan. I would check all of your chickens for possible mites. Usually a permethrin product such as Permethrin 10 spray is required to be used at least twice 7 days apart in the chickens. Then the coop bedding should be removed, and after spraying the coop, under boards and crevices, nest boxes and roosts, clean bedding is added. Mites are very difficult to get rid of and DE doesn’t work on them. Avoid straw or hay for bedding, since mites can be brought in on bales.