I am just now being able to research this--because it was such a shock to find my cat dead. That morning I woke up to get kids off to school. Looked for my cat as I always did. Making sure she wasn't locked in one of the kids rooms. Kolors had this thing where she would get socks, hand towels, rolls of tissue-- whatever she could hold in her mouth and drag it to the living room. (almost like she was moving a liter of kittens) I was picking her "liter"up and seen her on the bathroom counter in the main bathroom. I scratched her back said there you are fat cat, then walked out. That was the last time I saw her alive. OMG I wish I had picked her up and hugged her. Anyway I sat at the computer for about 20 minutes waiting for the kids to let me know they got to school then I went to go lay back down. I walked into my bathroom and I saw her in the dark laying on my bathroom floor. Which I knew something was wrong because Kolors never laid that way in the bathroom floor. She was laying on her side with her legs just kinda stretched out. (like she would look if she was walking) Before I ran out to get my husband I remember seeing a little bit of fluid at her mouth on the floor. and later I seen where she had had a bowel movement. And that's it. She didn't make any noises, she didn't cry-meow, there was no sign she was sick. In fact she had been fine the night before. When we got her as a kitten she was sick, so I had her checked out at the vet. They did the Fel. Aids and the Fel. Leukemia test. Everything came back fine, she had a resp. infection. And after we got her well we have had no problems with her. We had her fixed and de-clawed with no problems. She was just 7 years old. I say just because I had had another calico when I was a child and she lived to be 16 so I guess I am thinking that Kolors was young according to how long my other cat had lived. So I just don't understand what could have happened. I know that is was probably a blood clot. But I still just cant figure it out. Seems like there would be warning signs?
I am so sorry to hear of your loss, I know there is nothing I can say that will ease the pain.
As for what happened to your cat, well unfortunately without a post mortem it is impossible to say for sure.
However it could simply be a heart attack, a blood clot as you suggested or even a fit. My sisters cat had a fit in front of her eyes, he made no sound just lay on his side and shook for a little while and had an involuntary bowel movement and then just passed away. There was nothing anyone could do.
I must admit that it rather strange that your cat was quite young and had no history of ill health. But then again, we do hear stories of people who appear perfectly well just popping off without warning. So maybe your cat had a defect that was undetectable and unfortunately caused her death.
All I can say is, there is no point worrying yourself sick over this, there was nothing you could do and it does sound like she went quickly and peacefully.
best wishes Kate