That sounds like a respiratory disease. Many respiratory diseases affect chickens, so it is difficult to pinpoint the exact one. However, no matter what the disease, isolate her. Respiratory diseases often spread quickly, and can be serious.
Keep her isolated in a crate/cage/large cardboard box/small coop. Keep her warm, and give her electrolytes and probiotics to help improve her immune system. You can get special chicken probiotics, or you can give her yogurt. If her appetite slacks off (which sometimes happens with birds that have respiratory problems), entice her into eating using moistened chicken feed, scrambled eggs, and applesauce.
You may want to try antibiotics. If the disease is caused by a bacteria, antibiotics will help. Keep in mind, if using antibiotics, don't use probiotics or give yogurt. The best antibiotics for a respiratory infection are Tylan50 and Tylan200. Tylan50 is weaker than Tylan200, and is more commonly used for chickens. Tylan200 is considerably stronger, and can also be used. Either one will work well.
The dosage for Tylan50 is 1cc for large fowl, .5ccs for bantams, injected into the breast muscle (alternating muscles/injection sites each day) for 5 days. For Tylan200, the dosage is .5ccs for large fowl, and .1-.3ccs for bantams, injected into the breast muscle for 3-4 days. A good needle size to use is 22gauge; Tylan is not a thick antibiotic, so you do not need a large needle. There is a type of Tylan that you put in the water, but I don't know a lot about it, and it is more difficult to find. Also, antibiotics in the water take more time to work, and you can't be sure that the chicken is getting enough.
If you don't want to try Tylan, there are other weaker antibiotics like Oxytetracycline (sold under names like Duramycin, Terramycin, Tetroxy HCA-280, and LA200). Oxytetracyline is usually given in the water, for a week or so. It is not as strong as Tylan, but may be easier to find, so you might want to try it first.
For information on giving chickens injections, go to
http://shilala.homestead.com/injection.html. This webpage has good information on giving a chicken an injection.
Hope I've helped! Good luck with your bird!