Diarrhea and not eating are both common issues for cats. These symptoms can be caused by many different factors, including a change in diet, stress, parasites, or infections. It’s difficult to tell what’s wrong when a cat stops eating, vomits, or experiences diarrhea. The best thing to do is contact your veterinarian and seek the advice of a professional, since some serious conditions can result in pain, distress, and life-threatening complications.