Is your cat breathing noisily or faster than usual? Are they sneezing? Are they breathing through their mouth/panting? Any of these symptoms could indicate a problem with their lungs or airways
Cat flu can make a kitten extremely poorly and even lead to death. Speak to your vet about getting your kitten vaccinated against cat flu.
Cats can have allergic reactions just like people. Common triggers include insect stings, plants and medications (including vaccinations).