I don't care how skinny a horse is but if they threaten me over grain, they aren't going to get any and be rewarded for the behavior. I'd sit outside the pen with a lungewhip. When she puts her ears back and snaps at me, I'd smack her and let her know who's calling the shots. I might reward one ear forward depending upon the circumstances but I might also walk away with the feed if I feel I'm not getting anywhere. It's a dominance thing with her.
Even if I refuse to give her food she starts pawing and shoving the door with her nose and once when she tried to attack me I smacked her and she reared up and hit her head on the roof and that scared me really bad.