Agree with Saddlebag. Also I would encourage you to stop riding before this behavior starts. Even if it is just for one lap around the arena. Before you dismount give her a reward (treat, pat, kind word, whatever she likes), stand still for 30 seconds and then dismount. That way she will begin to learn that the end of the ride is not tied in to the behavior. Once you are more confident of her, try a little more, but if she leans back you're going to have to push her forward and get her out of it. A half a dozen more steps might be enough the first time or two. Just so she knows that you determine the end of the ride, not her.
All assuming it is indeed behavioral, which I do think it is.