Do you think you could post a video? It would be easier to tell what's going on from a visual.
Also make sure you aren't leaning. You may have the habit of putting more weight into one stirrup than the other and haven't even noticed, and since your horse is green (I'm assuming) she's very sensitive to that and it throws her off balance. This may or may not be the case, but it's near impossible to tell without seeing you ride.