You really should call the vet. Is there any possibility she could have gotten into something poisonous? Aspirin poisoning can cause breathing issues and lethargic behavior. If she was just lethargic and not eating, I would not be so concerned if it was just one day. Cats can get a flu like illness. But combined with the breathing, I would not take any chances.
"A kitten is infinitely more amusing than half the people one is obliged to live with in the world"- Lady Morgan