The mare was miserable. She was grumpy, grouchy, did not want to leave her stall, definitely did not want to be ridden...and anyone who touched her in the girth area on the near side got snapped at and kicked at.
Possible diagnosis? Ulcers.
Horses do indeed get stomach ulcers, and it is my experience that the primary symptom is uncharacteristic grouchiness. Ulcers are often caused by stress, insufficient turnout or too much grain.
There are various medications for ulcers. Feeding licorice or apple cider vinegar can also help horses prone to ulcers. Which suck for the horse - and the people the horse is grumping at.