Are you feeling okay, Mittens?
Cats are masters in the art of hiding their discomfort when something hurts. Their hunter nature makes them want to keep their weaknesses hidden from everyone.
Thankfully, there are many small signs that help you to identify when something is wrong or when something is bothering them. In order for this to work best, you have to know your cat well: what are their normal sleeping, eating, and playing patterns? Are they an active kitty, or not so much?
If you know their habits well, it should be easy for your to recognize if something is out of the ordinary.