Given that the horse had an “in depth pre-purchase exam barely a month ago and passed with flying colors,” and that the horse has been shipped recently, my first thought is an injury, possibly during travel.
A chiro is as good a place to start as any. My old guy was really off (sore, really pissy) after a long haul last year and a session with the chiro helped. But, if it doesn’t improve fairly soon, I think you need to break down and get the vet out.