It is difficult to understand if your cat is actually lethargic or just sleeping a lot.  Cats can sleep anywhere from 18-20 hours a day.  However, if your cat is acting lethargic and not eating, you should see a vet straight away as it could be something much more serious.  There could be a number of reasons that your cat won’t eat.  I’d recommend reading this article on the topic for more details:

https://www.vetbabble.com/cats/food-and-diet-cats/help-cat-wont-eat/

There are some helpful tips on encouraging your cat to eat, but again, if your cat is not eating because she is ill, it is important that you find the root cause of the issue and see a veterinarian for a proper physical exam.  They can palpate the abdomen and check other vitals of your kitten as well as a number of other things to make sure your kitten is healthy.