I *think* what may be causing this for you girls is that you may have a leg length discrepancy... See an orthopedic doctor and ask for a gait analysis and they will be able to tell if you are walking properly, if not you may need some orthotics. I am such an orthotics pusher
since mine have helped me and my DD so much. It could also be that your feet are pronating and you need better shoes too... Get it checked one way or the other...