I would have to agree with the others regarding a visit to the vet. Bloodwork and even an urinalysis is probably a good idea. Kidney disease, IBD, hyperthyroidism, dental disease, etc all come to mind. Check her water consumption, watch her litter box happens (frequency and volume), food consumption. If she does have some medical issue it is best to catch it early then to allow it to progress untreated. If the vet checks her out and all is normal then I would suggest visiting your local pet store and asking for sample bags of the foods they carry.