Medical Payments (Med Pay) will cover the medical payments of you and your passengers in your vehicle if they are injured in a crash. Personal Injury Protection (PIP) includes medical payments coverage but also will cover other documented losses such as your lost wages.
You may also be required to repay any benefits you receive under your Med Pay policy in the event you recover from the at fault driver's liability insurance. However, there is no subrogation right as to PIP benefits.
As for reimbursement of your Med Pay benefits, it may depend upon the specifics of your situation as there are legal arguments that can be made against repayment if you are not "made whole" by the third party settlement due to lack of insurance coverage.