What Causes Delay In Periods?

Usually I will get my periods earlier. (That is my last month period was on 16th and this month I should have got my periods before 16th, that should be on or before 12th but I had unprotected sex with my lover on 20th and 28 th of last month. I didn't get my periods still. It's been late for 4 days now. Please help me... very much worried.. what can I do. Please suggest