I don't know. I've got a 14 year old tom that's showing his age. More affectionate, sleeps on the bed instead of elsewhere, and like yours he spends way more time sleeping than he used to. Still, he seems content and in no distress. I think yours is somewhat young for this, but they're all different.
Anyway, no advice. You may take her to the vet for a checkup, but mostly give her the extra attention she seems to want.