I am so sorry for your loss, I can't even imagine how you feel! How very sad, she was so young!
The only thing I can think of is maybe she had a heart condition?
Sinister "The Prince of Darkness" 4 yr old black male GSD 3.11.09
Draco "Dakota's Unbreakable Vow" 5.5 mth old male Dalmatian 6.30.13
Cats: Chaos, Mayhem, Monster, Wicked