If her neck is losing control, I would be very tempted to say that it's possibly something that she got into - a toxin. It could be from anything - a mushroom she found, a puddle of water, maggots by a flower pot, a compost pile, etc etc.
You could take the conservative approach to that possibility by doing a gentle flush of her system to hopefully flush out whatever she might have gotten into. There are true flushes using either epsom salts or molasses and they're very strong and very stressful. The one I would use instead would be a combination of a little babyfood applesauce, a tiny bit of molasses, and water as her water for 4 hours. Then electrolyte water for the next three days. This will give her a little runny droppings so you will want her to get yogurt daily for the next week, probably some vitamins (which since she's ill I'd suggest anyway).
More on that below.
There is a condition called torticollis that can be associated with vitamin deficiencies, illness, and it usually is more of a stiff-necked issue. But as of yet (particularly because of the stress of showing, exposure to other birds and via the judge) it can't be ruled out. Either she could have picked something up (depending on how long ago the show was) , or had possibly been exposed to something and the showing experience just made her immune system drop and she is now showing the symptoms.
Or it could be an injury.
In all cases, Vitamin E can possibly help, will NOT hurt - nor will a well rounded general vitamin package. For those two, I would recommend polyvisol baby vitamins (the non-iron-fortified formula, vitamin section at grocers or
Walmart) 3 drops in the beak daily for a week. The vitamin E you would buy would be 400-700 iu's in strength, the oil capsules for humans. You will slit the end of the capsule and give her 2-3 drops in the beak daily for a week. Yogurt you'd give 1 tablespoon daily as a treat or mixed in with crumbles and water to make a damp (not wet) mash. I'd still give the vitamins separately to make sure she gets them, and I'd give them AFTER she eats a bite or two. There is also B vitamins in that vitamin which might help if this is a B vitamin problem, which I'm not convinced of yet.
I'm concerned that with the loss of control of her head that she might asperate if given liquids. You'll want to be very careful of that. Also notify us of any changes, like weakness of a wing or leg.
For the gentle conservative flush, mix a half teaspoon of molasses into a cup of very hot water. Stir to dissolve, and then cool. Dribble some of this into her beak if she won't drink on her own. If she will eat or drink on her own, you can use some more of that water to make a damp mash of crumbles, yogurt, 1/2 teaspoon applesauce for her to eat once. Then just regular crumbles dry or dampened with the molasses water - what ever she'll eat.
Only make this available for 4 hours, and let the molasses water be her only source of water until it's gone. Then water with electrolytes (pedialyte, or poultry electrolytes).
Try to keep her warm, calm, and quiet. I suspect her head going back and forth is her panicking because she can't control herself. But it could indicate something else.