She could have had a stroke. Dogs don't generally have heart attacks, but they do have strokes, and her symptoms fit that. A stroke is sudden, and it can happen to a seemingly healthy dog,especially an older dog. It is something that would not have been due to anyone's action or inaction. I'm sorry for your loss.
dont know hun i just lost my 11 year old beagle a week ago she was throwing really bad and the next day she died i have no idea what happened i know she wasnt poisioned bc she was an inside dog i feel ur loss as i am going thru the same thing its not easy but sometimes we miss signs that something is wrong and we dont know until its 2 late they are a part of our family