my pullet is coughing like a goose

My 12 week old Americauna pullet has begun coughing and intermittently holding her beak open to breath.  She sounds like a goose honking when she coughs. She is drinking and eating a little, but definitely seems to have lost weight. She is not flying up to roost.  Her poop looks normal. The other 16 chickens do not seem to be sick. They are housed in a protected run and warm dry coop in east TN, but currently Molly is inside the garage in quarantine.  I cannot see anything in her throat when I look, and I do not hear any wheezing or rasping in her chest.  I appreciate your thoughts!