Chickens don't get colds.I've been researching her illness and wondered if this could be Marek's? It all started with her not being sure legged and stumbling and then the sneezing and now the wheezing with heavy breathing. She can not stand up and lays on her side with her wings spread out. She is able to hold her head up. Could this be Marek's?
Sounds like a respiratory illness. If it is, most of them make the bird carriers for life and they can infect the rest of your flock. Some are also able to be passed through the egg to offspring.