My deepest sympathies to you on the loss of your beloved Sophie. :(
Whenever a young, apparently healthy dog just dies for no apparent reason my first thought is "bloat". Bloat can strike any dog but larger, deep-chested breeds are more prone to it. Essentially, what happens is that the stomach flips over on itself and switches places with the spleen causing major damage to both organs and the intestines leading from the stomach. It is always fatal if not caught and surgically corrected in time and it is so deadly that if a dog bloats at, say, midnight after the family has gone to bed, unless someone is made aware of it almost immediately, there's a better than average chance that the dog will be dead or close to death by the time the alarm clocks go off in the morning. It literally kills within hours.
Of course, the only way to tell what happened would be for you to have the vet perform a necropsy on her. Without that, there's really no way at all to tell what it was beyond hazarding a guess.
Again, my condolences to you on your loss.
I am so sorry you lost your baby. Did she get into any pesticides? Any type of poison? My prayers are with you.