My Dog died suddenly what happened?

My 10 year old Baby boy had a sudden episode of what appeared to be difficulty coughing. He quickly became distressed. Prior to this, everything was fine and he had just eaten his lunch a couple hours earlier.
He had a grade 3 heart murmur that was stable and as late as last week, his Vet said was fine and no fluid was in his lungs.
He kept trying to cough but seemed to have lost his strength to cough. I picked him up and loved him and sat him back down and realized he couldn’t walk. He could stand but couldn’t take a step. I took him outside and he urinated but still did not take a step. I called the ER Vet and was told to bring him in. Half way into a 40 minute drive, he craned his neck upwards and stretched his legs out while turning on his side. He was apparently in major distress. I finally got him back and calm and he was again lying on his tummy looking out the window. When we arrived to the Clinic, they took him from me and made me wait about 20 minutes.
The Vet later came out and told me his Murmur was now a “6” and he was going.
I went back and upon hearing my voice, he began breathing again. I began singing to him and telling him today wasn’t the day and begging him to come back to me.
For the next 2 agonizing hours he would stop breathing at times as long as several minutes then would restart his breathing on his own. He was clearly fighting with all he had. The Vet asked me if he had gotten into any rat poison which he had not. He had a lot of bloody fluid coming up and they were trying to aspirate it. They gave him several shots of Epinephrin for his heart. Finally he took one long last breath and was gone.
I don’t know what happened and the vet discouraged me from having any blood tests done after the fact. I can’t help but wonder if they could have possibly given him a shot of something before I got back as he had a port in his leg and there was a used syringe lying there. They said they had not given him anything.
While it doesn’t matter at this point, I can’t help but wonder if Vets will give a shot if they know the owner will refuse to euthanize. I just don’t know what the episode was or what caused it.
If anyone has any experience or knowledge I would appreciate your answers. He was fine then all of a sudden he was struggling for his life.