What Causes A Delay In Periods?

hi i am 13 days delayed i supposed to have a period on september 14,my last period was on august 14,2011,i also had a late ovulation on september 23,i had protected sex with my bf on august 29,he use condom and after he removed it to his penis he checked it if there;s any leakage and thank god its nothing but how i become delayed in my menstrual period,before i am regular but now 13 days delayed i am so much worried about my situation i don't know what to do.Please help me doctor