Cats for the most part are not forthcoming when they don’t feel well. They will give you clues… some more subtle than others, so it is really important to pay attention to any changes in their behavior. Something as innocuous as a change in sleeping patterns to litter box habits. I recently went to a clients house to see her dog and she mentioned that her cat was getting litter all over the floor and she had just cleaned it up and he had gotten litter all over again clue #1 and he was pawing in his water bowl clue#2 I went to look at him and he is a bit feral and normally runs away but this time he did not move and let me touch him ( clue #3). I asked my client how long this had been going on and she thought for a day or so. He is male so checked his bladder and it was really distended so I told her I thought he should go to the vet NOW. She brought him in and he was blocked had to have emergency surgery and was in the hospital for two days and made a full recovery. So definitely any change in litter box habits from a male cat need to be addressed ASAP because if they are blocked it can have pretty dire consequences; kidney failure and/or cardiac arrest, not to mention that being blocked is really painful.
The links below have good information on what to look for and what the clues might mean.. So have a little chat with your cat and make sure he or she is feeling purrfect.
http://www.petfinder.com/cats/cat-health/subtle-signs-of-a-sick-cat/
http://www.petcarerx.com/c/27/cats/cleaning-health/warning-signs-your-cat-may-be-sick