are you doing something wierd with your foot? Possibly tensing your ankle and rolling your foot in so you're riding with more weight in the pinky toe side? Make sure your stirrup is across the ball of your foot, with the outside branch more forward. There should be more weight in the big toe side of your foot so your pinky toe will be up a little higher. Make sure you aren't shoving your heel down, just keep your ankle relaxed with your weight in them. Also make sure your stirrups are the correct length.