I had a dog that got sick suddenly once. It was back in college and I was a dog breeder, and went up to see him quite often.
I noticed it as soon as I opened the door to let her back in the house she didn't come to me but was just lying on the ground. Turned out her stomach flipped and was cutting everything off, including her air; she was having problems breathing and moving around.
They stapled her stomach to each side of her rib cage and she ended up being ok. But losing a pet always sucks. Sorry for your loss.