Liz-   In addition to the suggestions Dr. Kellon made, you may also want to have a horse chiropractor check out your horse.  Per my horse's chiro guy, having dental work, especially if a speculum was use, can get the jaw and poll to go out of joint and cause pain.  Yawning is a symptom.   At least that was one symptom I saw with my horse.  I now make a chiro appointment about a week to 2 weeks after dental work.
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Ronelle and Yoyo
2015 Bend, Or, US