some good ideas some i’m not a fan of – as a trainer i mostly run into this problem with
1- a rib that’s out – first, as many have suggested, get her body checked, chiropractor or equine osteopath.
next most common issue is as has been visited by several,
2-improperly fitted tack (saddle) and your horse knows that with the final tightening it’s going to hurt, hence the message, don’t hurt me (the warning bite)
3-i teach all owners of horses i train to take no less 3x to tighten the girth – first one is just enough so the saddle doesn’t slip, mess about doing other things, tighten a second time what can be done EASILY, all tacked up, take a walk BEFORE you mount up, one more tightening before putting a foot in the iron.
i am not a fan of feeding while tacking up, i’d rather they be paying attention to me however in the quest for figuring out if it’s an old bad habit – that might be an interesting thing to try as if she’s busy eating and doesn’t bite then it’s an old habit, remove the food distraction and go back to re-training with the take 3-4x to completely tighten the girth.
i know ulcers can make them girthy but it’s easy to check the body (bones and muscles) first, tack/saddle, slow things down and if none of those help check for the ulcers, i haven’t found this to be a root cause, but that’s my experience, again, it’s been bones/tack/poor handling.