Oh my goodness, I am so very sorry for your loss
I am glad that at least you were with him at the end and he wasn't all alone. These little ones are so very quick and can eat things without us even noticing, sometimes. That's the only thing I can think of, is that somehow he ate something poisonous. It sounds like it happened very very quickly, What did the vet say?
Thinking of you
RIP Chauncey