I am so sorry that she is doing worse. Has she been taking liquids? Obviously, since there are other sick chicks still with the breeder, this was something that came with the chick. It could be brooder pneumonia or aspergillosis a fungal infection from mold in wet bedding. It also could be infectious bronchitis or mycoplasma, but I would get a necropsy done on the chick if possible by your state vet, if the little one dies. Her body would need to be refrigerated in a plastic bage, and then take or shipped to the state poultry lab or college. Are your other chicks doing alright?