You did so much for her. I'm not a vet or even have much experience, but I do know that we all have a beginning and an end, and you gave her a beautiful life for 15 years. It sounds like she was in total system shut down. This is hardly ever due to just one thing and doesn't reverse with one treatment of one symptom. She very well could have had an underlying condition such as cancer. If anything, you made her as comfortable as she could have been and the bute helped control her pain.