I can relate. I lost my 10 year old, very active, very healthy cat to kidney disease 6 months ago. He had just had a checkup and everything was fine. One day he suddenly stopped eating and we took him in. He lived for a week and then we put him to sleep. I cried for days afterwards. It was devastating. He was the last of my pets I would have expected to die suddenly. He was a member of our family.