My dog suddenly got sick and died one day. Why?

I had an amazing dog names, Alex, that was seven-years old. Alex had to be an outside dog since I moved in with my cousin, and one morning I went to check on him and noticed that he wasn't energetic like he usually was. He didn't even walk up to me, and that was totally unusual! I noticed that he had suliva around his mouth, he hadn't eaten, been throwing up, and it had blood in his poop. I ended up staying with him all that morning and keeping him in the house and made sure he had water, but he still wouldn't eat. Once he finally started moving around I let him outside to use the restroom since he was going to the door, and when he went pee I noticed it looked like nothing but blood. He walked a few feet, and lied down. I had no way of communication since the internet, cable, and phones weren't working, so I went to a neighbor and called for help. After a fews of holding him and being with him, I ran inside, left the door open, and out pants on since I was dressed up, and when I came back I saw he threw up, moved, and was shaking violently. I am sixteen and that day I happened to loose the closest thing to be since I was nine... A week and a half after my birthday. I can't stop thinking about what happen, but what I would really like to know is what happened.
It's been over half a year and I never figured it out, but I sometimes feel at blame and miss him incredibly.
What are any possible reasons to why he suddenly got sick that morning and passed on?

[Misfits4Me]

Poisoning can cause what your describing......I'm sorry you had no help to get him to a Vet.....My condolences on your loss.....RIP Alex......Karla

[Caryopteris]

I am very sorry for this loss which has continued to trouble you.  

There are all sorts of things that can go wrong with our pets.  I had 3 dogs get incurable diseases at 7.  One died quite suddenly.  It was very upsetting and I wished I'd been able to figure out what was going on and helped her.  I did have autopsy (actually they call it necropsy) performed, and found out it was an inoperable brain tumor.  So at least the mystery was solved for me.  I understand that not knowing is part of the issue for you, wondering if there was anything you could have done.  It could have been something she ate, whether it caused poisoning or perforation of her digestive tract.  We will never be able to completely protect our pets from everything, but at least you loved your dog and she had a happy life with you for 7 years.  I am sure she loved you very much and didn't mean to leave you so early.  Life is like that, unfortunately.  

[Margot49]

Oh, such a sad story and sorry you lost your dog.  It does sound like some type of poisoning.  If the dog was outside, it could have gotten into something like a poisonous plant, snail bait, even a rodent that ingested poison and your dog at the animal.  Just no way to tell for sure.  You will probably never know.  I'm sorry for your loss.