I am so sorry. Without a necropsy you will never know. You do know that she had a good life, full of your love and care and that she leaves you with wonderful memories to cherish. The good memories will come soon to overide your devastation
13 is a good age for such a big dog. I know it's hard for you but if it were me I'd try to be consoled that my dog did not linger and suffer from some debilitating ailment. This is hard, I know. My hairdresser lost her father at age 82 when he was hit by a car. She did not have a chance to say good bye but she acknowledges that is how he would have wanted the end of his life to be, sudden.