it sounds like she has gape. these are small worms which live in the respiratory tract and is invariably fatal i am afraid. you need to move her away from your other chicken as it is highly infectious and the likelihood is that she will have it to. isolate the sick chicken, get something from the vet (worth a try but you have to be realistic) and you will to thoroughly clean/disinfest the coop and any ground that they have been on. The worms live on the ground so this is why you need to do this. suggest you google gape for more info sorry to be the bearer of bad news but gape is horrible and she could go on for many days like this, moving less and gaping more in which case i am afraid you have to do the right thing for her sorry.
One of my girls (down to 2 at the mo) is not 100%. She is eating and drinking well but clearly has a slight respiratory infection. She is opening and closing her beak sometimes - this is what happened before I lost my 1st girl. I have added some hay to her nest box to keep her warmn at night and she is apple vinegar in her water. Any thing else I can do?
Also struggling to give them enough green at the mo? Any tips?