Does she have any other symptoms or problems breathing? Heat exhaustion and dehydration can be common in hot summer months. Coccidiosis could also be a problem in younger chickens. A regular vet can do a fecal float on a few fresh droppings brought in, to look for worms and coccidiosis. Corid is a good treatment for cocci, and is used in the water. Symptoms of cocidiosis are lethargy, not eating, diarrhea with mucus or blood, and standing hunched or puffed up. I would treat the sick chicken with electrolytes and vitamins in the water, and give it with a dropper. Offer some chopped egg and regular feed.  Check the feed for any possible mold, which can cause severe illness. If she should die, I would refrigerate her body in a plastic bag, and contact your state vet for a necropsy.