I saw This video on YouTube awhile ago and he had the same thing going on and started treating her for respiratory infection, but then treated her for gapeworm. He’s a chicken pro, so you’ll most likely find it helpful. Good luck! Post an update when you can
Aww poor girl 😢
Could be gapeworm but fortunately if it is it’s treatable. I would separate her from the flock if she’s distressed and treat her for worms specifically gapeworm, and treat the others as well incase they’re all about to be gasping for air too...