To check her crop: Keep food and water away from her tonight, then first thing in the morning before she eats or drinks anything, feel her crop (on right side of her breast below her neckline) It should be completely empty. During the day and until roosting time the crop should have food and water in it and can feel soft, hard, squishy and all kinds of ways. But first thing in the morning it should be empty. If it is not empty there is a problem.
Where in the world are you located? Is it summer or winter? A sick chicken must be kept warm. If it is cold where you are can you bring her inside? If it is warm have you any way of separating her inside the coop? To figure out what is ailing her you need to be able to monitor her poop, water and food intake, and laying or not. Lots use a dog crate and place it inside the coop. (Left in the coop you don't mess with the pecking order).
Don't worry, much more experienced people will be along to help you out. Best wishes!
