I am so sorry for you. It sounds a lot like a cancer, hemangiosarcoma that many of us have faced. There are some threads on it but the tiredness and the rebound followed by rapid death. If it is, there would not have been much you could have done......
Heartworms take years to kill a dog with a lot of signs of problems.......no it was not heartworms
NAPWDA Certified Cadaver Dog
Waiting at the Bridge (italics
=GSDs) (hemangiosarcoma=blue):Grim , Cyra, Toby
, Rainbow, Linus, Oscar
, Arlo & Waggles