The change in behavior you're describing - sleeping more and eating less when she's normally playful - is worth getting checked out, especially since it's a noticeable shift from her usual routine. Cats do sleep a lot, but a sudden drop in appetite combined with lethargy can signal anything from a minor issue to something that needs attention, and at 7 years old it's better to be safe. I'd call your vet and describe what you're seeing; they might want to examine her or at least give you a better sense of whether it's worth coming in. If she stops eating altogether, seems in pain, or gets worse, don't wait.