Cats flick their tails slowly and in an upright position when they are content. However, if they begin to quickly thrash their tail or twitch their tail in a lower position then they are most likely upset and irritated by something.
Cats also dilate their pupils and flatten their ears when they are upset. If a cat is giving this signal, it is a sign that all humans and animals should leave the cat alone. When cats are upset, they tend to like to stew in their annoyance by themselves. They often retreat to a favorite piece of furniture and stay there. When human owners go over to the cat and try to engage it, they may be scratched or hissed at.