Pug died suddenly.

Aloha,
Our beloved Pug Tobi passed away unexpectedly on Wednesday and as much as I want to let it go...finding reason I believe will help with the healing process. Here's what happened:

My wife as she usually does a few times a week took Tobi and our daughter for a walk. Tobi is overweight so we were in the process of helping him lose the weight. This walk was a little longer than normal but Tobi was able to walk the entire distance. As we usually do after walks we bring him in, let him drink water, and he usually just lays on his stomach to recuperate as he's breathing heavily. About an hour later I checked up on him and noticed there was poop all over the floor, which was unusual since he doesn't poop in the house unless we're away for an extended period of time. The poop was soft and wet but not what I would consider diarrhea and it was not bloody. Not thinking much of it and considering he was too tired to walk I picked him up and carried him outside under a cool shady area for him to lay down as I cleaned the room with the poop. As I carried him I noticed a piece of poop fell out of his butt onto the floor. Again I was unsure what to make of it. I noticed as I carried him that his stomach was wet but figured it was from the water he was drinking.

We left him out there for about and hour checking on him frequently. We misted his body with water to help him cool down and he seemed to get better as his breathing wasn't to rapid. We brought him in house as night approach and laid him on a blanket. His breathing was deep but not rapid. We noticed as we would massage his body the his muscles would twitch and about 20mins later he stopped breathing. I rushed him to the animal hospital but it was too late...

Upon doing some research heat stroke seemed possible but he didn't have any signs of vomit or bloody stool. Another was he had a heart attack due to him being overweight and the stress of that walk on that day. Still we are unsure and very much heart broken knowing at some point we should've just rushed him to the vet...hopefully you can help us find some sort closure... Thank you in advance!