Kitty must be too tired
Cats communicate largely through body language, and the tail is one
of the most important parts of your cat’s communication toolbox. By
understanding “tail talk,” you can understand how your cat is feeling
with just a glance.
A happy cat, for instance, walks
with her tail held high, and a super-happy cat will add a quiver at the
tail tip to demonstrate her joy. A mildly annoyed cat will twitch the
end of her tail, but if she’s lashing her tail back and forth, you’d
better step away, because the claws are about to come out. A cat
concentrating on prey will have her tail held low to the ground,
although there might be a very slight twitching at the end as she tries
to control her excitement. Source: Catster