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I think that it was her time to go and she went. It sounds like there was more going on than you or the vet knew about. If you want to know for sure, you can request a necropsy (autopsy) and have the vet review the findings with you.
Just keep in mind that you did nothing wrong in this situation. You sought the help of a vet and followed his instructions. There's nothing to feel guilty about.
I hope this helps to soothe your pain a little. I am very sorry for what you are going through.
You are more than welcome. I know that you are going through a lot right now. If you don't know the story of the rainbow bridge, let me know and I'll share it with you.