Just like people, cats can become stressed out. This can happen when bringing your cat to the vet, traveling with the cat, or simply because the cat is neurotic and wants to make your life miserable.
Since cats have better senses than humans, one way to calm a cat down is to appeal to its senses. Keep it around familiar smells and keep harsh noises away. Also make sure movements around the cat are slow and watch your cat to see if it wants to be touched or not. By catering to your cat’s stress levels, you can ignore the fact that you might be just as stressed out as well.