I am so sorry for your loss. I have to agree with Jocoyn it sounds like hemangiosarcoma.We lost our 12 and half year old GSD Daisy to it. As Courtney said the only true way to know is a necropsy. We lose to many of our beloved breed to this dreadful killer. Again my condolences.
Miss Chevy Cruz - 1/25/2013-1/29/15
Thunder -1/25/13 -7/25/15
Lucky-GSD -Rescue -2/16/03 - 03-21-16
Charlie- GSD-Rescue - 7-4-12