I agree, have your vet check her teeth no matter what her previous owners say. Ask your vet if she has wolf teeth. If she does, have them pulled out.
A couple of days after her teeth are floated, ride her again and check her comfort.
If she's still unahppy, then what kind of kimberwhick are you using? Single joint, ported, roller middle, uxter sides, smooth sides, etc. If it's an uxter kimberwhick, with the slots for your reins, which slot do you have the rein in?
Unrelated to the bit, is she sensitive to flies or gnats around her face? She may be reacting to the flies or gnats, and not necessarily the bit.
It also could be just a training issues. You may need to work through it.
All that said, I would definitely have her teeth checked ASAP. Then you can consider the rest of the possibilities.