This is a common complaint and warrants investigation. Teeth grinding is a manifestation of discomfort. While it isn’t specific for oral pain I would recommend starting with a good oral exam to ensure that there is not an underlying dental or other medical reason in her mouth, to explain her behavior. Sharp enamel points, a cracked or broken tooth, and wolf teeth can all lead to discomfort. If the oral exam is unremarkable other explanations should be sought.
Stress can lead to gastric ulcers and affected horses will often grind their teeth. Gastroscopy is the definite way to diagnose gastric ulcers. A flexible, fibre optic camera is inserted through the nasal cavity, down the esophagus, and into the stomach to allow direct viewing of the lining. Typically horses are sedated for this procedure. Treatment of gastric ulcers consists of 28 days of omeprazole (Gastrogard) and a follow-up gastroscopic exam is recommended to ensure complete healing.
Interestingly omeprazole can also be used to prevent ulcers (only if no active lesions are present). Ulcergard (1/4 of the treatment dose) is used for this purpose. Finally, an assessment of your horse’s full body comfort should be undertaken to ensure that the teeth grinding isn’t related to soreness elsewhere.