Weight changes in cats, whether gain or loss, are usually reason for worry. Either of these symptoms could indicate a more serious health issue. Short-term weight loss is more important, while long-term weight gain is usually more dangerous. Schedule a vet appointment for a check-up or at the very least a weight check if you are worried about your cat’s weight.
Even if it is modest, lethargy is an indication of a problem. Contact your veterinarian if your cat appears to be sleeping more than the normal, lying around a lot, or has a low energy level in general, something is most likely wrong with your cat’s health.