Well apart from doing the checks (back, teeth,saddle) as everyone has mentioned it could of been evasion because your instructor is making him use himself properly...which is harder work, bit like us using gym equitment correctly...hence having a strop, horses have different personalities just like us, well to certain extent, some put up with stuff, some like to express themselves more, from affection to down right naughtiness. Unless you horse is doing this all the time, then I would not worry yourself too much, there will be times in your lessons when your horse objects, but yes certainly get things checked out so then you know its not associated with pain, then you just need to tackle his attitude....but I'm sure it will of just been a one off.