this is really bothering me

I had a 10 year old shitzu. She was laying on our bed and sneezed really really loud a couple of times. I notice a slight blood tinged fluid that came out of her nose. I called a vet , not my regular one as he was away and the emergency vets charge so much here that it is really hard to use them. This vet said she had a slight fever and thought her teeth were an issue, she proceeded to give her a rabies shot which I learned later is not wise if they are ill. she put her on antibiotics and said in four days get teeth cleaned . She had a bad heart murmer also I was told. She seemed very sick very quickly after the sneeze she had stuffy bloody junk from the nose, and then got a little better, I took her to my vet who kept her and gave her meds and fluids,by I’ve. She was sick three days and went into seizures and we had to have her put down. Supposedly her blood work was alright and also she had a check up for senior dogs and blood work 2 months previously was told a slight heart murmer. This has really bugged me as if it was infected teeth the antibiotic should have helped. They said she may have had a sinus or brain tumour, but did no xrays. I wanted an autopsy but they did not seem to want to bother. I feel guilty as she was fine and then with the sneezing it all went down hill any thoughts on this would be appreciated it has been 8 months and still bothers me. Donna