My cat is lethargic, breathing a bit heavy, and won't eat or drink?

You don't want ANY fleas, they carry Lyme Disease and all sorts of nasties, and all it takes is one, so try get the cat onto Frontline or something like that (not Hartz).Lethargy is a problem, since it's a symptom of so many things, some serious, some not so serious. Keep an eye on her. Feed her wet food with a bit of water mixed in to make sure she does not dehydrate in the meantime, First thing in the morning, off to the vet.