And like isamonster says, cats are experts in hiding pain - in the wild, show weakness, and you'll be attacked and eaten, so it's their instinct to hide their pain.
A cat wandered into my house one day, not even limping, and gave it some milk (didn't know better back then not to feed other peoples' cats, or of lactose intolerance) Cat was purring it's head off the entire time, but when I saw the other side of the poor cat, it's leg looked like it had been through a mincer. 
Rang OOH vets, who were brilliant, and luckily the cat was chipped, so the owners were contacted, and gave permission for amputation, the only option, the leg was ruined.
That's just an example to show how well cats hide pain. They purr for many reasons, and one of those is to self-soothe.