ryn24, I'm so sorry.
I know the shock and sadness you must be feeling. Something very similar happened to my cat last month. Her health had been declining slowly, but she had had recent bloodwork done and there were no warning signs. I went out for dinner and when I came back 3 hours later, she was gone. Still in her little bed on the loveseat. Aside from her head being tilted over the edge of her bed and some drool, absolutely no signs of what might have happened. She also still felt warm to me, and I rushed her to the emergency vet, but she was already gone.
It doesn't sound like your kitty choked on something if you didn't see any signs of distress. It could have been an undetected heart condition, though my vet said that the most common cause of sudden death in cats is a blood clot. He believes this is what happened to my Margaux, and the circumstances of your kitty's passing are nearly identical. He would not have suffered, and I hope you can take some comfort in knowing that.
I kept thinking the same things you're thinking: if I had come home sooner, if I had checked a couple of minutes earlier, if I had missed some sign...I am still struggling with all of the unknowns, but trying to remember only what the vet said: that it would have been quick, and that she wouldn't have suffered. It's so hard, and I think it will be impossible not to always wonder just a little bit, but please try not to focus on the how. As you see from Straysmommy's experience, even an autopsy can't always give a concrete answer. It's one of the reasons I chose not to do one.
If you feel up to it, feel free to post in the Rainbow Bridge forum. You will have a tremendous network of support here, because so many of us know what you're going through.
Sending hugs and wishing you lots of strength for the hard days that are inevitable.