It's hard to say from just that; but I would recommend a vet visit. It could be any number of things; from the quality of her diet, food allergies or illness. The vet should really be your first stop though. Do you have other cats? Is she Indoor or outdoor?
Has she been spayed yet? If not; this is the time to get that taken care of as her hormones will start kicking in soon. I would start asking around and see if you can find a vet to give her a well exam and any vaccinations required as well as spaying her (if not already done). Over the internet; we just cannot replace the advice of a licensed veterinarian. It's very hard to say from limited information; we can share experiences but we cannot physically see her.
When did she begin sleeping a lot? Was it a sudden change? What sort of food does she eat? Any changes in her environment, foods, etc? Those are just some questions to start with.