It could be the beginning of a respiratory disease. Have any new birds been introduced to your flock lately? Or, have you gone to any poultry shows? Those situations could have introduced a respiratory disease into your flock, or it may just have been carried in by the wind, wild birds, etc.
If it IS a respiratory disease, the question becomes whether it is viral or bacterial. I'd start giving her Oxytetracycline (sold under names like Duramycin, Terramycin, Tetroxy HCA-280, and LA200) in her water. If this doesn't work, you could try Tylan50 or Tylan200. The dosage for Tylan50 is 1cc for large fowl, .5ccs for bantams, for 5 days. The dosage for Tylan200 is .1-.3ccs for bantams, .5ccs for large fowl, for 3-5 days.
If the disease is viral, the only thing you can do is keep the bird warm, well-fed, hydrated, and stress-free. Isolate her, and make sure that she gets enough to eat and drink. Putting probiotics and electrolytes in the bird's water would also be a good idea (but don't give probiotics when giving antibiotics).
Hope I"ve helped! Please keep us updated!