PAIN SIGNS BEHAVIOR FOR CATS

Decreased or no urination in the litterbox. This is very important and requires immediate attention. If you can find no sign of urine in the litterbox or elsewhere, let your veterinarian know immediately. The cat may have a urinary blockage, which can be fatal (particularly in male cats) within hours.  
Decreased or increased urination. This is a potential sign of medical issues (such as kidney function or diabetes) Take your cat to the veterinarian.
Urination outside of the litterbox. If this is a new behavior, the cat may have a urinary or bladder infection, urinary crystals, or may have pain elsewhere. This can also signal stress, insecurity, or simply not liking something about the litterbox, but medical reasons need to be ruled out first.
Defecation outside of the litterbox. There’s usually a medical component to this behavior, whether it’s just a touch of constipation or diarrhea; there are also other, more serious causes. See your Vet.
Decreased or increased thirst. This can be an indicator of changes in kidney function, diabetes, or other medical issues and should be looked at by your vet.
Aggression. Aggression can be the result of many things: fear, stress, or pain. A cat who is normally even-tempered who exhibits aggression (or aggressive body language) towards you or another animal in the home should be evaluated by a vet.  
Avoidance or sensitivity to being petted or handled. This could indicate pain in a particular part of the body or generally not feeling well. As a note, most cats develop arthritis and joint pain as they get older, but they don’t need to suffer—relief can come in the form of medication or other treatments.
Restlessness, inability to settle. A cat in pain may have trouble lying down, sitting, or getting comfortable. They may relax for a few minutes then get up and pace or shift to a different position.  
Change in gait, limping, or difficulty jumping. This can be indicative of injury to soft tissue or bone, arthritis, or other medical issues.  
Purring or excessive vocalizations. Cats who are in severe distress will often purr; the vibrational frequency of purring has been shown to be beneficial for healing tissue and bone, but it may also be an attempt at self-soothing. Further, vocalizations can be a sign of pain, cognitive decline, or the need for medical attention. Many medical conditions, if caught early, can be treated or reversed.