Check inside her ears for any kind of sores and/or ticks. Ticks love to get in a horse's ears and they can be a real pain to get out because they are hurting the horse and you're trying to pull them out
She could also have dry skin and it's flaking in her ears.
Sometimes horses will get little wart-like bumps in their ears that itch. One of my horses had them when I bought him but, amazingly, they disappeared after I changed his diet
Can you get Witch Hazel? Here in the U.S. it can be found in any drug store, WalMart, grocery store, etc.
When one of my other horses had bug bites in his ears, the vet had me wet a cotton ball (not dripping wet), gently massage/clean his ears, then gave me a medication to wipe in there.
Your Mini may not appreaciate you poking around in her ears. Try to do it in a gentle massaging way and don't make eye contact with her.
After I've said all that, the head shaking could be something else. Hopefully others will come in with more thoughts
If you can't get it figured out, then I would call the vet