We've had chickens with allergies. Move them to a different area and the symptoms disappear. It could be a respiratory infection. It could be Infectious Bronchitis. It may be Mycoplasma. It could just be dusty bedding in the coop or mould. First stage is, on an urgent basis, to thoroughly clean the coop. Watch the hen for swellings around the eyes or bubbles. I agree with Stereo. Antibiotics is a last resort and won't treat IB or Myco anyway. If the coop clean doesn't sort it and there are no other symptoms she has a respiratory infection and will need antibiotics. Problem is they get permanent lung damage and you needed to have tackled this problem a week ago Jayr2.