I'm so sorry, Mark. Acute problems like this are devastating. No warning, nothing you can do and you always end up second guessing and thinking what if.
My very favorite golden of all time, Amanda, was slowing down but she was 12. All of a sudden she collapsed, got pale and shocky. I got her to the hospital and she died before I could even get an IV in her. I know it was a hemangiosarcoma tumor in her heart that ruptured (autopsy), but no evidence before hand. Still second guess myself on that one.
Again, very sorry for your loss.
Doug