Late period! Please help!!

Okay so my periods usually come pretty normally. And I just had my yearly pap done on Dec. 8th. I was due to start around the 25th of December. I'm not on birth control yet. I was waiting till I start my period to start taking them. Also there is no possible way I'm pregnant since I just had sex for the first time on new years. Is there a reason I could be this late? Thanks in advance for all help!