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.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.