I've been doing full (regular) pull-ups for 4-5 months now. I never really did more than 7 reps per set, but my form has always been proper before. Today, during pull-ups, I noticed that my left arm was doing most of the work and that I was moving my wholebody towards the left side all the time. I always knew that my right hand had a weaker grip on dead hang and hanging leg raises, but what happened today has never happened before — this is the first time ever. Can it be that I had overtrained my right arm or maybe even slightly injured my right shoulder or something?
We can't really tell you if it's a muscular imbalance, or an injury, or overtraining, because... How could we possibly know? We know exactly as much as you've told us.
What you should be doing (and this extends to ALL exercises), is to perform the movement symmetrically. If you find yourself leering to one side, because one side tires before the other, it could be a muscular imbalance, which is corrected by training both sides equally.
DO NOT be tempted into training one side more than the other. You'll only create new imbalances. Keep the exercises equal on both sides, and let them fall into place.