By nature, felines avoid any tip off that they are experiencing pain. It relates to their survival mentality where the jungle cat who exhibited any weakness became prey for the stronger cat.
Today's knowledgeable cat owners will verify their domestic cat carries inherited traits of their jungle forefathers and have never forgotten the urge to hunt, stalk, and survive.
The trait of hiding pain makes it extremely difficult to detect discomfort, particularly in its early stages. Early indicators can be subtle including consuming less water and food, a small change in attitude, less desire to be in the family mix (too much time spent under a chair observing), low energy, and perhaps even mild lameness.
The progression of pain will often lead to more obvious signs such as lack of grooming, as a result of the discomfort incurred when bending their bodies to lick and clean. Perhaps the cat that greeted you each morning on the kitchen counter by the coffee maker is now on the floor at your feet. As the need to hide pain increases, you could note more time spent in another room alone and less use of the litter box due to the difficulty getting into the box and the body positioning needed to defecate.
The message is BE WATCHFUL. If you see any indication that there is a change in your cat's normal routine, consider this a potential sign for pain. Your first call should be to your veterinarian for a complete examination. Do not self-prescribe any medication. Cats are extremely sensitive to drugs and they have very few safe options in pain control, even short-term. If you do detect signs of pain in your feline, caution should be used when palpating or examining. The most loving cat will bite and scratch when you examine the painful area.