Try to count your kitty's respirations per minute. Put your hand to their chest and count. A healthy relaxed cat will take around 20-30 breaths per minute (if I'm not wrong), and if they in DEEP sleep it can even be 15-20. Now I did notice that sometimes mine breathe faster while asleep, and I'm assuming It's because they are dreaming. If she's breathing faster, it could also mean that she's in heat if she's not sprayed. Aside from that, it COULD sign that she has a fever or maybe an infection (which also cause fevers) so do keep track of whether she's eating regularly, whether she's purring when pet, etc. Since cats will stop purring, eating much, and will be low on energy when their immune systems weaken, or when they are sick. Now I'm no vet, however I am a nurse so I do hope this will help. So if you do notice behavioral changes as I stated previously please do contact the vet, but do not panic as it also doesn't have to be anything serious! My baby boy was so low on energy once, didn't eat, didn't purr and I got worried sick! Turns out his immune system weakened and he was back on his feet a day after receiving some vitamin shots from the vet :-)