The fact that it can be narrowed down to a specific movement, that isn't repeated on the opposite site, would make me think pain. If the saddle fit doesn't work (assuming teeth and feet are up to date), I'd get her muscles checked. Baring in mind though, a release of tense muscles can change the fit of the saddle.
If that doesn't work, there are a host of other problems it could be. Only recently, I had a horse that had issues turning on one rein and it turned out that it had arthritis in its hock which made it difficult to step under and support itself on a turn. It could also be a physical problem with yourself. An issue with your body could be blocking the horse and making it uncomfortable to turn when she has to compensate for you.