I think this warrants a vet call. Lethargic kittens not eating or drinking are cause for concern, hairball or no. It doesn't take as long for a kitten to suffer the adverse effects of not eating and drinking as it does an adult cat. It can also be hard to tell a hairball hack from a respiratory cough, so I'd suggest calling your vet and asking if you should bring your kitten in, especially if she's still lethargic and not eating.
