Please accept my deepest condolences Danson .
Nobody here will be able to tell you what happened with any degree of certainty though. It's all just guessing. Sounds to me like the dog may possibly
have had some type of obstruction, but that too is a guess based only on what is written and my own personal experiences.
I can tell you that a former adopted cat of mine had 2 obstructions from eating a mouse and a bird. Both times the cat's behaviour was off within 24 hours and the cat saw the vet within 24 hours and both times x-rays did reveal blockages (showed bones) that needed surgical removal. Each surgery was over 2000. The cat had 3 of them during his life. Most expensive cat ever.
The only real way to confirm what you believe is through an expensive autopsy.
Personally, I agree with Jull who wrote;
"I would say remember him with all those good memories you have left, and hold on to them tight, don't hurt your self wondering what if, what if... I know if I was you I would be feeling the same way you are, but I hope you can find some comfort in knowing he loves you and is looking after you "
Remember the good times.