It is completely normal for breast implants to shift in the early post operative period. It may take a few months before this stops being as noticeable. Also implants placed under the muscle are more likely to be felt moving since the pectoralis muscle is attached to the arm so any time there is movement of the arm there is potentially movement of the pec muscle. This in turn can cause the implant to move. In any case movement is normal and will be less evident over time.