How old is she? Is she laying eggs? Hens can suffer from many different illnesses, some related to reproductive illnesses such as internal laying and egg yolk peritonitis. Check her crop for normal emptying by morning, look for mites and lice on her skin around the vent and elsewhere, look at her breasrpt and keel for loss of weight, or tightness or enlargement of the lower abdomen. Has she been wormed recently?