It doesn't really matter whether or not you think thats not the problem. Horses need their teeth looked at at least once a year. My gelding rode just fine, could run a barrel pattern at Mach 5, be calm and quiet all the time. Well we had his teeth looked at for any signs and he had a massive abcess that we never would have caught.
I'm not saying this is absolutely, 100% the problem....But I just want to emphasize how important it is. Some horses are so good natured and forgiving that the signs are incredibly minimal.