When a girl is having questionably protected (or unprotected) sex and NOT trying to get pregnant, her periods are a cause for celebration, just like pay checks are for those who are poor. Hence, she's living period to period instead of pay check to pay check.
Girl #1: I just got my period!! That means that all that sex I was having while I was drunk must've been protected!!!
Girl #2: You really need to stop living period to period.