Our vet said with our cat, (and our previous dog- who did the same thing except with barking,) that she did not appear to have any pain, but she was confused and frightened and weak, so did not feel safe alone and was probably crying out for comfort. My dog had his first episode of confusion and weakness last month. He's fourteen, and again, the vet said there's no signs of pain. I did get painkillers, just in case, but they had no effect. He just wants to be held. So I second checking it out with a vet, but it seems to be pretty common for animals to cry for help towards the end of their lives when they are probably not in pain.