It wasn't the injections, this started long before the injections.
Besides he already had those injections about 1 year ago and he was fine.
This kind of reminds me of what happened to Marley in Marley and Me, the dog's stomach flipped or something? It's kind of a spontaneous thing I guess, happens if a dog is really active right after eating a lot. I don't know, I'm not an expert at all I'm just throwing it out there.
Anyways sorry for your loss, I don't have any pets now but I have had pet dogs and cats die before and I was very attached to them. It's really too bad, that looked like a great dog.