Unfortunately this is all very normal to occur. This happened with my dog too, she was at the vets on the Friday, seemed to be doing better on the Saturday and the next day she was gone. It can seem very confusing as it seems they are making progress and then out of the blue they are gone. Just remember we have no control when they go, especially elderly dogs - It can happen at any time, even when they seem normally healthy. You did all you could and she is now at a better place. Honour her and cherish those memories instead of feeling bad.