What Causes Delay In Menstruation Other Than Pregnancy?

Hi. I had unprotected sex on the 13th of this month he did not ejaculate in me, my period was due on the 16th and it is now 6 days late. My period always comes on time an lasts around five days. I have not been under any stress as college and work has been going great lately so I can't blame it. Do you have any idea of the reason for my period being late. Thankyou.