I'm not in any way equating this to your situation, but I had this exact thing happen with our first dog. He was about to turn 10. Our dog was in great health and had never had any issues to this point.
I was having a beer with a friend one afternoon and I was going on about how great of a dog we had and said "But he's getting older and his time will come, I suppose. Some day we're going to have to say goodbye." Got up the next day and he was acting weird. We had to say goodbye a few hours later due to hemangiosarcoma (cancer that causes a blood vessel to burst resulting in internal bleeding); 100% fatal.