Are you prepared to take her to an avian vet? Have you brought outside birds into your flock recently? If so then you may have brought home a disease as the birds brought into your flock could be carriers but are not showing symptoms of disease themselves. If you caint take your hen to the vet, I whould seperate her from the rest of the flock and try to treat her. From what you described it could be mycoplazma or corzoya? Maybe infectious bronchitis? Its hard to say so I whould start with tylan injectable or souluble as a starting point you can get this at a livestock supply store or livestock pharmacy.I whould also monitor the rest of your flock and if they start to show symptoms go ahead and treat them all,just remember that your birds will alway remain carriers of whatever they may have and if your okay with that then you should practice biosecurty and keep a closed flock. Also any new birds you may want to add will most likley get this and you will have to treat them. This may be a bad situation but you can try to treat your birds and make the best of this situation also don't bring birds from other farms or auctions into your flock as you may get other disease ontop of what you already have in your flock. Its best to buy new birds from a reputable hatchery of incubate your own eggs good luck with your hen.