Searching for Answers

I was the proud owner of a wonderful little pug who had adrenal cushings until this past Friday. He was 12 and had some arthritic problems but otherwise all of his tests were great. Then Thursday night he developed a wet cough that was making it difficult for him to breathe. I totop him to the emergency vet and they began antibodies due to what they thought was pneumonia. However he continued to get worse. In the end he could barely breathe and started bleeding from his nose. As I didn't want to watch him suffer anymore we released him but I am a wreck and have so many questions and what ifs. Does anyone have any idea what could have caused this??