Can you post any pictures? She may have been pecked or she could have a fever. Is her lower belly enlarged, or has she lost weight in her breast area and overall? Is she laying eggs? Many hens her age will suffer from reproductive disorders, such as internal laying, egg yolk peritonitis, ascites (excess fluid in the lower belly,) and cancer. I would separate her in a crate with food and water. Offer chopped egg along with her chicken feed, and they sometimes like it when you add water to their feed to make it wet. Diarrhea can be common with most illnesses. Is she low in the pecking order, or kept from food by the others?