If her abdomen is hard and large compare to the other hens, then most likely she has an internal laying/reproductive disorder. Symptoms include loss of weight, not eating, swollen/distended or fluid filled abdomen, loose droppings that can look like egg, difficulty breathing and lameness. Antibiotics may give some relief for a short period of time, but usually long term they are not successful.
Do the best you can to keep her hydrated.
Without testing, there's no way to know what is causing her decline. AI usually presents as a respiratory illness.
