Hi there,
This certainly sounds like your dog came into contact with some external toxin or poison. A young healthy vaccinated dog would not go downhill so quickly with the common bacteria and viruses, so I suspect it ate rat poison or a similarly potent toxin (of which there are many, albeit rare).
Rat poison is an anticoagulant, so it makes the dog bleed internally and also causes blood to appear in the vomit/stools. A dog would inappetant and lethargic in this situation, and would urgently require the antidote (for rat poison this is vitamin K1) in order to survive.
Another possibility is that your dog could have swallowed something sharp which perhaps penetrated the wall of the stomach or intestines, causing contamination of the abdominal cavity with bacteria, leading to life threatening peritonitis.
I am sorry to hear of your loss. If you are prepared to go to any length to find out the cause, you can have a post mortem exam done. They can test for various toxins, and can also look at all the organs individually for signs of disease. However, they might not find anything anyway, and it can be distressing for the owner that the body of their beloved pet has to be dissected fully during the examination.