[Mercedes]

Agree with the above poster...make sure skin is not getting pinched, make sure the cinching process is done over several minutes and the final tightening occurs after the horse has been walked in hand a little bit. Also, ulcers can be the culprit.

If that all checks out, then next have a chiropractor in to check as they can have a rib out.

If that's fine too, then he may simply be a horse who's very sensitive in that area.