Dog Allergies Treatment

We understand the dog is 13 years old, Doberman Mastiff. Our best friend, truly. A young Hungarian girl, who had squeezed through a padlocked gate, came to our door, bruised and shaking, with a Hungarian English Dictionary; she said “help me.” She didn’t know anyone here; we helped her. Her divorce from this Romanian man was finalized Friday. Tuesday at 2am someone is in the yard. Our dog is acting sick, farts, Whew! Coughing, choking, sneezing, bloody stool, but the Vet says allergies. Next day the dog throws up food we didn’t feed him. Thursday morning the dog can’t walk, can’t breathe, I lay down next to my best buddy, hold him tight, and he takes his last breath in my arms. It hurts so badly. What happened in three days?