Oh how utterly awful- the not understanding why would be making me crazy I think. I am not recommending this but just posing this as a question - might this be the type of situation where an autopsy or at least talking with your vet in great depth would be useful. A good vet understands that they are there to help the human too as much as the animal patient.

Is there any possibility he could have gotten into anti freeze or something toxic. Please be kind to yourself - there was no way you could have known - it may very well be that it was something like a congenital heart condition and that nothing could have been done anyway. You just don't know and that may be the most difficult part of it. My sincerest sympathies.

Very sorry you lost your cat. I have no idea what could have been wrong - he could have eaten something poisonous, ate something that caused blockage, maybe he had cancer or a heart condition - it could be lots of things. Try not to blame yourself. You sensed something was wrong and you made a vet appointment. Wasn't your fault his condition was so serious that it needed immediate attention, but you couldn't pick up on it. Even if you did get him to the vet that day, theres still a chance it was too late for them to save him. Lots of people (myself included) would not have understood what was really going on there, and lots of people would not have called a vet to begin with. They would have just waited a day or two to see if things improved on their own. If he didn't have any obvious behaviour that pointed to posioning, like throwing up, seizures, etc - how were you supposed to know what was wrong? You gave him love when he really needed it, and it's obvious you truly loved him, or you wouldn't be here now writing about it. He knew you loved him, and that's what matters. Sorry again that this happened, but one day you will see him again. Don't feel guilty - wasn't your fault.

Ditto re: the advice to not feel guilty. If it had appeared that death was imminent, or that his condition was at the very least, dire, you would have rushed him to the vet, appt or not.

I am so sorry for your loss. I hope your other kitties give you extra love and cuddles, and vice-versa during this sad time in your household. (((HUGS)))

I'm so sorry. I just loss one of my cats and I know exactly how it feels. You did nothing wrong. You noticed a issue and made an appointment. It did sound like how many cats act with simple infection.

In between my cat passing and visiting my vet for more answers, I read lots online about sudden death. Here's the things that came up again and again:
1. Poison
2. Heart Worm
3. Cardiomyopthy

If your cats are like mine, one getting into something wouldn't be possible without the others. I ruled that option out for my own house pretty quickly.

For heart worm and cardiomyopthy, I would go talk to your vet. I became so distrustful of my vet for a few days after my cat passed, but I felt much better after I just went in there and talked to the vet for 20-30 minutes. If you go to the same vet regularly, just go in and say my cat suddenly passed away and I have a lot of questions. That is what I said and there was no charge or anything.
The vet will likely explain the common reasons for sudden death and if anything else is needed to ensure your other cats are still healthy.

In the end, my cat concealed his illness very well until he started to lose weight just like many cats. Cats are not like dogs. If something is wrong, they may conceal it for weeks or months without ever giving a single clue until they are very sick. You can't blame yourself though. You did everything right.

I am so sorry, that is awful. We lost one of our dogs this way a few weeks ago. She had just been diagnosed with congestive heart failure that afternoon and put on meds, and when I got up at 4 am the next morning to give her one of the meds, she had passed away in her sleep.

Thank you all for the kind words and techigirl78 and petra I am so so sorry about the loss of your cat and your dog. I have spent every day since just spoiling my remaining cats.