Today I went and rode my mare. While I was walking her she would pin her right ear and tilt her head toward the middle of the arena and she was shaking her head. What could be the problem. I've only ever seen her do this one other time.
If she lets you, you might try to look in her ear to see if there is something in there that you can get out. If there isn't anything that you can see and she continues to do this, I would take her to the vet asap. We had a dog that would carry his head cocked off to the side with one ear dropped and come to find out, some grass burrs had gotten into his ear and were working their way through his ear drum. The vet had to put him out and dig in there to get them out.