Oh, Amy!!!!!!!!! I'm so shocked about Nina...I just can't believe you lost her like this!!!! I'm so very sorry...I kept up on your posts when she ran away, and also when she had her beautiful kittens.
She was a lovely girl, and was loved tremendously by you, and by many on TCS. You did nothing wrong by taking her in to have her spayed...once in a great while, a cat will react badly to the anethetic...this is not something you could have predicted, or prevented. You did what any loving cat owner would have done...you did the RIGHT thing.
This breaks my heart for you...I'm crying right now. Stay strong, and know that she is giving you cheek rubs, whisker kisses, and head butts from across the bridge.
Again, I'm so terribly sorry for your loss.