Depending on where you are located, it could be the heat. Chickens cool themselves by panting and holding their wings out from their bodies.
Another issue could be a respiratory infection. Do you see any crust around the nostrils?
Plus, since this is a Cornish X, they are known to have trouble in heat, in times of stress, and when they get too large. Sometimes, even at a young age, they have heart problems and even heart attacks. Sometimes with electrolytes in it will do well. You can use something like Gatorade, Pedialyte, or a vitamin/electrolyte supplement from the feed store.