What Causes Delayed Periods In Spite Of Taking Birth Control?

today is monday, not this saturday but last saturday my bf and i had sex prtected(but the condom got stuck inside me) and twice unprotected.. neither time did he ejaculate inside of me. As a precaution i took plan b about 5 hours after the first time we had sex that night. i still have not received my period.. should we be worried? were freaking out!