Is rib out of place? Cracked maybe? Abscess? Peritonitis?
Before I would pop her with toe of boot I would have vet out to do thorough exam, not a walk around one either.
That said, how is she with everything else? Is she bossy about other things, or is this the only time she is telling you to quit something?
Your riding mechanics could be off too, and she is trying to keep from being in gait that you will be jouncing around at and hurting her mouth.
Video would help, and also analyze if there are other things going on, and report back. By that, pinning ears when grooming, not getting down to roll, flinching when being groomed, ill when tacking up.
Horses make me a better person.