Actually, the photo of her and the look on her face and in her eye? I would expect exactly this type of behavior from her.
And feed/hay could be too much for her...when combined it could be too hot. May also be getting more hot of feed she was previously fine with. I've seen that happen.
And I don't care whether a horse has on halter/lead/full tack or is spinning around on a merry go round, it darn sure better not have an attitude with me at any point.
Your horse is spoiled. Whether or not you are part of the problem? Can't say, don't have a video of what you are doing, the tone of your voice, or how you move.
But the others, the treat feeder especially may be the root of this. Even one time of moving back in reaction to a horse that is inclined to be like this, will imprint in their heads that a human will give way. They will then escalate until they are doing exactly what you describe her.
Hay should be analyzed by vet to see what it is like, and possibly rethink feed too.
What exactly is she getting? And agree that 5 flakes seems a bit much too. Even a stalled/dry lotted horse shouldn't need that much, unless you are heavily working her.
And if you rely more on having some sort of "control" over horses, instead of your "inner core", that gets you in habit of being more worried.
Do any of you use the "soothing...it's allll riggghhhhht" type voices? Or are you more matter of fact and common sense people?
Horses make me a better person.