Please examine your chicken all over. Look for external parasites (lice, mites). Look at the chicken’s vent. Does it look clean and healthy? Like someone else already said, feel her chest. Make sure her crop is empty in the morning. Feel her breast muscle and see if her keel bone is sticking out excessively. Is she laying eggs? When was the last one? Is her abdomen swollen and squishy at all? Is she breathing normally and quietly?
It’s possible she is ill, but also possible she’s having a juvenile molt, which makes them feel lousy. Is she dropping feathers? Does she have any bald spots? Any prickly spots with pin feathers?
Sorry about all the questions. Just trying to help narrow it down.