I'm so sorry to hear this. You're doing the right thing. I've had to put down 2 of my cats and it never gets any easier. It's up to you if you'd like them to do a necropsy(I think that's what it's called). My former vet did one on my RB Pepper for free because he was very interested to see what had caused his sudden decline in health. He never found anything significant. RIP sweet boy. Please take care. (((HUGS)))