Welcome to BYC.......
It sounds like your girl may have gotten a respitory illness or is having a reaction to dust etc...if you don't think it to be dust... You can get Tylan 50 cattle injectable med and either dribble about 1/2 cc if she is a full size not bantam hen, in her beak. It works best if you inject it under the loose skin..but not everyone is comfortable with that. Do this twice a day for 3-4 days and you should see an improvement. If another chicken comes down with it... You may want to research respitory illness in chickens ...