I am so so so sorry for your loss. On July 23rd I had a similiar sudden death of my Peanut man who as perfectly healthy at 4 1/2 years old.
Here is what happened to us and I hope this does not upset you however it helps knowing you made the right decision as he never would of been the same. After I read your tragic story I now wonder if Peanut hit his head on the ground and the same thing happened.
Monday evening I returned home from work as usual and let everyone out to run in the back yard. They all ran together barking and yipping as they always did and for some reason buddy got a bit snippy and yipped at Gio in a nasty way. I scolded him and then he proceeded to do the same to Peanut. Never once nipping him or even touching him. In the next moments everything happened so quickly it was like a nightmare. As everyone ran to take care of business I turned and Peanut was on his side. I grabbed him up in my arms quickly and he was limp like a rag doll. His tongue was purple and back in his little mouth. I put my finger in his mouth and swiped his tongue to the side and then proceeded to do CPR and nose to mouth breaths with no response. Only sound was a slight gurgle after a few moments and I now know what that sound was and will never forget it, it was the death rattle of my poor little Peanut Mans last breath on this earth. I continued to do CPR and nose to mouth breaths for several minutes and there was no response. I knew then my little guy was gone. I wrapped him in a towel and took him to my vet who assured me that I did everything possible to help him. But what took my little guys life in that split second I will never know. I refused an autopsy as I just did not want to put his little body through that and heck what good would it do? It certainly wouldn’t bring him back. Peanut was healthy and happy and had no previous history of heart problems. There was no blood, no assume no head injury just that horrible stare that I will never forget. My vet seems to think that he must have had a heart issue that was undetected and it was his time to travel on to Rainbow Bridge. Again I am so sorry for your loss and please reach out to me if you want to talk.