That's a rotten feeling to have but I wonder why you got that worried about it. Had he been really skittish all the time or only when you were cantering. If it was when you were cantering it could be that you were doing something which made him worried. The trouble is they pick up on our worries. I would try to sing a little song in rhythm with his footbeats or something - a calming song to help you and that will help him.