Welcome to the Cat Forum, I am so sorry it had to be under these circumstances though!
Unfortunately, I can relate to your loss.
I adopted a 2 1/2 year old calico in November, 2004. Everything was great! She got along right away with my other cats and altho she had a little cold, she was getting over that. One Saturday AM I woke up, was playing with all the cats and she was happily running through the house. I remember even saying to her, "well you just fit right in don't you?! Are you happy here?"
I went to run some errands and was only gone a couple of hours. I got home and she was lying motionless on the bathroom floor not breathing. I ran her to the emergency vet down the street but she was gone.
I did the same things I am sure that you have done... going over the day and the days leading up to the loss - trying to figure out if I missed something. I will tell you that from my reading and the vets I have spoken to, they believe it was a heart attack, which while not common, is certainly possible. they could have done an autopsy, but I figured she was gone so it didn't matter. But that is always an option that may give some answers.
From my research after her death, sudden death in cats is most likely due to a heart condition or heart worms or possibly an aneurism of some sort.
There is not a lot of info but here is a link that was helpful. http://www.vetinfo.com/csuddeath.html
The loss of any pet is terrible but when it is so sudden and unexpected, it is so devestating!!
While I can't give you answers, I pray that you find comfort in knowing you gave the little one a good home for the short time he was here.