Hmmmmm

We've had a few days off due to the weather. We're having typical april weather with some very heavy showers and trying to avoid them is quite difficult.
Managed to ride Remy today, but it was a bit of a strange session.
We started off in the field warming up in walk and then went into the school for a little trot work and hopefully a few walk to canter transitions.
The trot is becoming looser and better balanced, but I noticed after trotting on the left rein, that he started to nod (upwards) for a few strides on returning to walk. I don't mean throwing his head during the transitions, it was after the transition as we started to walk on, and it stopped if I straightened him or put him on the right rein.. He doesn't do it coming back to walk on the right rein.
I've noticed a reluctance to give me correct bend on the left rein, the last few sessions, preferring to keep slight outside bend, and put it down to a phase - natural stiffness/crookedness. We've had periods where he's been better on the left rein, the right rein being the more difficult.
When he lifts his head, it's like he's trying to take the rein away, and if I let him stretch right down he stops lifting his head, although we still have this slight reluctance to bend left.
It may be bit/teeth related - he does have a blind wolf tooth, which I didn't think was causing him a problem - or maybe he has tweaked a muscle and is reluctant to stretch the outside, or he has pain on the inside foreleg - he seems to lift his head up when the inside foreleg touches the ground?
Is it something I'm doing - with my hands - or perhaps just my extra weight in the saddle?
I didn't have a lunge line in the school, so couldn't actually try him on the lunge, but I did attach the reins to the noseband to take the bit out of the equation, and tried riding him that way. The nodding was still there when we returned to walk, but much less, and obviously the bit was still in his mouth, so it could still be having an effect on the left rein.

He's just had his teeth checked, so I've contacted the osteo to come give him the once over. Also, I'm going to lunge him tomorrow to see how he goes, firstly fully tacked up, and then without the saddle and then the bridle. I always lunge him off the training cavesson, but I will completely remove the bit to see if he's any better.
I'll see how he goes on the lunge, if he looks fully sound. He doesn't feel lame, but to me, head nodding, however slight or occasional, signals that he has an issue or pain somewhere.