I am sorry to hear you are worried about your Chesapeake Bay Retriever. My name is Rebecca. I am a veterinarian and will do my best to help.
Is she taking anything for her stiff legs, besides the Adequan? How long has she been getting the shots?
Is she vomiting? Is she drinking water?
I have done some research and loss of appetite has not been reported in dogs on Adequan, so there might be another reason for her losing her appetite. Have any blood tests been done recently, to check her kidney function, etc.?
If you look at her gums, do they look a normal bright pink or do they seem pale at all?
The main reported side effect with Adequan is abnormal blood clotting and bleeding so you see things like pale gums, bruises on the skin, etc.
Since she is 13, and not eating, I would be doing a thorough physical exam to palpate lymph nodes and internal organs, listening to her heart, checking blood tests if that has not been recently done, especially if you stop the Adequan and her appetite does not return.
Please let me know what other questions you have.