Ahh sad.
I was lucky with my Franky cat, we were able to spot his discomfort before it became untreatable: a tiny stone blocking the urethra, resulting in fast (within a couple of days) destruction of the kidneys and consequent death by blood poisoning if not immediately treated.
Yet in the very initial stages, bar some odd motions (pausing mid stroll), he exhibited no discernible signs of suffering.

I think animals are more like that than us humans, they'll poke up with pain for longer than we will, before telling us "This Hurts!".