What Causes Delay In Periods?

Hi, may I answer your health queries right now ? Please type your query here..                                                               i had sex with my boyfriend on the 12th may and my period was suppose to come on the 23rd may and it hasn't even up to now but he didn't break in me and i'm not on any contraceptive but i do have exams approaching soon and i know that stress is a cause of late period but it has me freaking out more but today i took a large dosage of vitamin c hopping that it helps any advice