I am guessing hemangiosarcoma too. I have lost three to this. It is a cancer of the spleen or heart *which grows very rapidly and then it ruptures and the dog either dies immediately or has a bleed, gets tired, recovers, and the cycle repeats until the end. It is very common in the GSD.
From what I understand it is not thought to be particularly painful
I am sorry for your loss. Take some comfort in the rapid death, and that you were lucky to be there at the time. There is not much that can be done. Even a splenectomy typically only buys a few months and is a risky surgery at that.
*Grim had it in the shoulder, not the spleen but it still progressed rapidly. Toby had several bleeds over about a 3 month period and Cyra was fine one day and gone the next with no warning.
NAPWDA Certified Cadaver Dog
Waiting at the Bridge (italics
=GSDs) (hemangiosarcoma=blue):Grim , Cyra, Toby
, Rainbow, Linus, Oscar
, Arlo & Waggles