He did in fact get his teeth done (about 2 months ago actually, I didn't even know!) so I've ruled that out.
So I'm taking his feet and ears back into consideration. He had shoes on last summer because he has some problems with his sole not being thick enough. I took them off when Winter started and he was fine. Now that it's warming up again he's been a little off, so I'm going to put the shoes back on. The head shaking started before his shoes were put on the first time however, but has gotten worse now that they're off.
It's gotten really bad to the point where he flails his head up and down at the canter (very, very annoying!) and will NOT put his head down for me. He just locks his jaw and pulls (he's an English horse btw).
So I'm stuck again :/ I was thinking about maybe switching his bit, but I don't see how that would be the problem since I've been using the same bit since I got him and he's been perfectly fine before.
Any ideas as to what it could be?