Get thee to a veterinarian Diabetes, kidney disease, hyperthyroidism, lymphoma, liver disease... these are things she should be checked for and ruled out before just increasing calories is the answer. While you are at your vet, ask for some a/d (or whatever brand they have's "recovery diet") once you know your diagnosis. a/d is a high calorie food fed to hospitalized patients that can help your kitty regain weight. And I will say for all of those diseases there are very classic symptoms, but I have diagnosed plenty of lethargic hyperthyroid kitties and diabetics who were drinking/urinating normally and eating very little.