
If she wasn't spayed perhaps it was pyometra or some similar infection.
But........ in the end it doesn't matter why does it? Its horrible and the vets do sound a little slow off the mark but knowing that wont change a thing either. However it was quick and she was at home surrounded by everyone that loved her and not perhaps at a vets on a drip for a day and going in the night anyway but away from you...
Nothing will make this better but the pain becomes less sharp with time and the loss more bearable - and one day you can just have the happy memories again instead.
My thoughts are with you. Anyone who has lost a dog unexpectantly or swiftly will have thought of the whys and what ifs - its normal - but just know you did nothing wrong and loved her every minute she was with you and there can be no blame or guilt in that.
Hugs to you and yours - give your other furbabies that extra bit of loving to help heal both them and you.