I'm very sorry for your loss; it's so hard to lose a beloved companion. Remember that you gave her a great life, and that whatever caused her death was likely completely unpreventable and untreatable.
There's no way anyone here can diagnose your bird's cause of death based on your description and an unrelated YouTube video.
Birds hide their illness, and by the time something like this happens, it's already too late.
Please remember the good times, and channel that into positive action, whether it's caring for your other birds (if you have them), or other areas of your life.
I think this video may help you, too: