If it were pain, he'd be off in his trot. He's been like that before if ridden a day or so after getting his feet done or just from stomping flies sometimes. It's really just him being too smart or trying to get out of working. He's fine walking once we've trotted or cantered some and on trails he's fine too.
Not necessarily true, pain can come out in a number of ways. Drastic change in personality is one of them. I wouldn't totally cross of pain if he is already sensitive. I personally would rather rule out pain first with my vet, then start really getting on his case if he is in fact just not listening.