I agree. I would first have the vet check her out. THEN if they don’t find anything, I would have a crop in my left hand and for every time she turns around just tap her on her muzzle, while continuing to tighten the girth. I have had one try to bite me when I tighten mine once I get on. I always carry a crop and while you don’t have to use it often, at least the horse will learn not to bite.