I am so very sorry for your lost. Having lost my Sammie last month I understand your pain so well. Had she been ill?
From your description it reads your dog passed peacefully - about the best we can hope for our pets and ourselves. Death is always a shock and sad and hard to accept...but it happens to all and best for those who survive that passing it is best to celebrate the friendship/love as we mourn out loss.
Blessings to you and your lost pet,
I am so sorry. It sounds as if she died in her sleep, which would have been peaceful for her. But when a good well-loved dog passes away it is always so painful. I miss my good girl to this day, and always will. I loved her best, so I too know ow you must be feeling right now....
My heartfelt condolences.
Unless she had been suffering from any illness prior to her passing, then it would be hard to know what killed her. Only an autopsy (necropsy) could give the answers, but you may not feel like doing that, and wish to lay her to rest peacefully.