She had very similar symptoms to what you describe. Cancer had pretty much taken over and it was a rapid decline. Muscle mass had deteriorated in hind legs. Refused to get up when I got home from work. Started having issues with going to the bathroom involuntarily. Labored breathing. It was obvious she was in pain. I just talked with the vet and made an informed decision from there. My deciding factor was "is extending her life by a few days, maybe a couple weeks good for her?" My conclusion was no. I felt I was being selfish by extending it. We had a good run together. Not sure if that helps, but good luck in your decision.