I agree with everything everyone has said. You should always tighten the girth in increments. I would put it on as loose as possible, walk her around, tighten a second time, and maybe work her a little on the ground or continue walking, and then before I mount, tighten one more time. I have often seen a horse be slapped on the side in preparation for tightening the girth. I would NEVER recommend that. Also, regarding the potential for ulcers: you hopefully can find a vet that will do free ulcer scoping. Sometimes they work with the pharmaceutical company to do the free scoping. Take advantage of that if you have it in your area. And finally, just check to make sure there are no girth galls, tick bites, etc., which I’m sure you’ve already done. She may also have a sensitive stomach which might call for some pre and/or probiotics.