Have you checked her comb, eyes, breath, poo and feathers? Does she smell differently to the others, and is there any discharge around her eyes, nostrils and vent?
Discoloured comb, watery, dull eyes, smelly breath, runny, hard or strangely coloured poo and dull, dry feathers are signs of illness, as is any discharge.
In any case, you should isolate her straight away, as diseases spread, until you know what's wrong with her.
It could be Marek's disease (Range Paralysis). It's caused by a virus belonging to the Herpes group, and it affects mostly young chickens. The visceral type of the disease causes internal tumors on the liver, kidney, spleen, gonads and occasionally heart. Your chicken is more likely to have the neural type, which causes the progressive paralysis of the wings, legs and neck, usually from lesions on nerve trunks.
While Marek's disease can progress incredibly quickly and cause sudden death, the lesions can also be eliminated and lead to complete recovery. I would advise you to take her straight to a vet, because you will need to identify quickly whether it is Marek's disease or not, as it is transmitted by inhalation and soon all your chickens could have it. It might be an idea to take the others along too to be checked for signs.
There is no treatment for Marek's disease. If you think it will go away by itself, then you have to keep the chicken isolated and let the disease run its course. If it seems there is no hope, it would be best to have your chicken put down.
Good luck, I hope she does get better!
