Does her face smell bad? I am worried that is sick with either MG or possibly coryza if she smells bad. Coryza can cause thick smelly nasal discharge, cough or wheezing, and gasping. The other chickens were probably already exposed by the same source as she was. If she does not smell bad, start some Tylan 50 injectable, but give it to her orally. Dosage is 1/4 ml per pound 2-3 times a day for 5 days. Does she weigh about a pound or less? Try to get her drinking and clean her nostrils with cotton and water or saline.
If she has a bad odor, you can use Tylan, but I would also get her on a sulfa antibiotic such as SMZ-TMP, Albon, or sulfadimethoxine. You can get it here https://www.medi-vet.com/Bird-Sulfa-Sulfamethoxazole-Trimethoprim-480mg-p/19328.htm
or through a vet.
If she has a bad odor, you can use Tylan, but I would also get her on a sulfa antibiotic such as SMZ-TMP, Albon, or sulfadimethoxine. You can get it here https://www.medi-vet.com/Bird-Sulfa-Sulfamethoxazole-Trimethoprim-480mg-p/19328.htm
or through a vet.