What Causes A Delayed Menstrual Period?

help! i dont know how to count of how many days im delayed.. last jan. 15-21 i had mens, but until now i still dont have mens?last jan.13,i had sex but its withdrawal method..only one round of withdrawal,and 2 rounds with condom.. im so confuse.. ais there a possibility that im pregnant?and how long could i wait to check it?i also suffer from diarrhea this week and dont know if its related to it?tnx pls reply asap