Unexpected death of a young dog

My 1-year-old puppy just died unexpectedly a few days ago. I didn't get any answers from the emergency vets, just a guess, and I can't understand how this happened.

I rescued him a few months ago, and since then, we've been to the vet multiple times to make sure everything was alright. Every time I noticed something different, I would do a check just to make sure. But I never did anything more complex than that, so I have no idea if he had any heart or respiratory issues (or other diseases that need an X-ray, etc.).

I wasn't home when he died, but I was told he went for a normal walk, like every other day, then suddenly started coughing up some blood and collapsed. That was it. I can't understand what could've happened. He seemed really healthy and full of energy. Sometimes he would cough once, very occasionally, or gasp for air for a few seconds, but since it wasn't a common event, I didn't think it would be something concerning. He had an upset stomach a few days before this happened, but I already called the vet, and he was taking probiotics + special kibble and getting back to normal.

Could it be something undiagnosed? I never caught any signs that could lead to this...
I'm not asking for the real diagnosis, because I know you can't give me that without a necropsy, but I just want to know if this could be a natural reason, because the amount of guilt I feel is unbearable.