Head tossing and ear shyness- PLEASE HELP!

My sister is trying to sell her horse and she's currently on trial at a barn. However, just before she went on trial 3 weeks ago, she developed some kind of ear problem that's been causing her to toss her head and she's essentially un-ridable right now just because she constantly seems uncomfortable while being worked. Obviously the barn won't take her if this continues to be a problem, and we really REALLY need to get her off our hands.

The problem more specifically:
She tosses her head constantly at the canter- other gaits are fine.
She will immediately jerk her head away if you touch her ears.
It started about 3-4 weeks ago. It has happened once before but it only lasted about 3 days or so.
Initially we assumed she had some parasite or bug or something, but a vet has looked at her ears and said there was nothing wrong with them. We are getting a second opinion tomorrow though because I'm pretty convinced there is SOMETHING the vet missed the first time.
The other barn's trainer also thought it might be allergies, but allergy medication hasn't made a difference. Another theory was that she didn't like the wind, since the slower gaits were fine. However, a bonnet thing didn't help either and she's sensitive about her ears being touched, and that's not really a wind issue.

Oh, and another thing- the problem started before we moved her to the barn where she is on trial, so the change in environment isn't what caused it.

If anyone knows what this could be, I would be so so so grateful if you guys could help me out. We really don't know what we're going to do with this horse if she doesn't sell.


THANKS!

I'm thinking there may be some cause that the first vet didn't find, so getting the second opinion might be a good idea. I will say though that one of the mares at my stable did the same thing. She wouldn't allow me to put on her halter because it involved touching her ears. She didn't have any issue other than she just liked being a jerk.

Check out the entire head - not just her ears. Maybe she smacked her poll on something, and she's tender between the ears as well. Maybe it's in the jaw and radiating upwards towards her ear area. Her poll or neck may be out of whack and need an adjustment.
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Thanks guys, I'm really hoping the new vet finds something else. I'll be sure to get him to check out her entire head

It sounds like ear mites to me but the vet definitely needs to verify that. Ours uses liquid ivermectrin and squirts it down into the ear. Hopefully your second opinion can determine what's wrong with her.

How well hidden are ear mites usually? We can't see anything when looking down her ears, and the first vet didn't seem to do a very thorough inspection of her ears either. I feel like that's the most likely though, just from looking around online

I agree with the others that a second opinion is a good idea, and to check for pain elsewhere in the head that might be radiating. Definitely thoroughly explore those issues. One other thing to check for, only because I have seen a similarly mystifying situation with another horse showing the same behaviors, is to check bridle fit very carefully, paying specific attention to the width of the browband. I know a thoroughbred mare who was grew very ear-shy and would lose it under saddle, and it turned out that her otherwise TB-size head was just a bit too wide for her horse size bridle and she was very sensitive to that. It's not likely to be the cause of this horse's issues, but it's worth looking into if you're still stumped after a second opinion.

Well my horse has had them but I never saw them so I'm not sure what size they are but I know their small and easy to treat. Has the vet been out for the 2nd opinion yet?
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I know you checked for bugs but check again down as far in her ears as you can see and make sure there are no ticks. My horse started having problems where didn't want his ears touched. I couldn't even bridle him! I started checking his ears but didn't see anything the first time. I figure ,maybe like JustDressageIt said, he just hit something but after a week he still wouldn't let me get the bridle on so I started checking again (in better light this time) and found 2 huge ticks in one ear. And a couple weeks later found a HUGE and I mean huge one way down in his ear! But once I got them out he was fine. And to keep the ticks out a rub petroleum Jelly on the inside of his ears.