I am so sorry you lost your
dog and at such
a young age. I have lost
a couple
of dogs also in past years suddenly and it is always a shock. One had a large tumor but no signs of illness, came home from work to find her in shock, rushed to the Vet but lost her. I did have a necropsy done and that is how the ruptured tumor was discovered. Unfortunately, unless the blood work indicates something specific like liver, kidney, pancreas, poisoning, etc. you may not know.
So sorry for your loss.