Call your vet out ask him to assess your horse on the ground and check it over, if there are no probs get your vet to check when you ride along with the instructor, you may also want your saddle checked. Once you get the all clear and 100% sure there is nothing wrong lunge your horse to make sure he knows the word canter, then when you are riding to a long collected sitting trot before the canter ask your horse to flex to the left and the right through his neck then when he least expects it ask for canter, however reduce the amout of outside leg you use and increase the inside leg and say ''canter'' keep trying until you get the transition. Hope your horse is ok, and some of that helped.