Do you have a lot of wild birds that use waterers or feeders near your chickens? I would watch for any new symptoms. Antibiotics would only be effective if she has a bacterial disease. Is she able to eat and drink okay? Is there any gasping? Did you feel of her crop to see if it is full and puffy, firm, or empty? Watching for new symptoms, or for other birds having symptoms is key to how to approach it. Here is a good video on the different types of respiratory diseases in chickens by a vet school: