Can a girl get pregnant if she get her period a day after she had sex.. please respond ?

Unlikely. : If it is really her monthly period, and not another type of vaginal bleeding such as what can sometimes happen after sex, it is highly unlikely that a pregnancy would result. That being said, it sounds like there may be a need for a more reliable contraceptive method. The "rhythm method" is notoriously unreliable, and should not be counted on to prevent pregnancy.