What Causes Delayed Periods?

i was due my period on 28th of march 2011,i had unprotected sex on the 11th of march and on the 13th of march when i wiped myself after going to the toilet there was a light coloured blood but nothing after that,i have had pains as though i am going to start my period but i have had nothing as yet,i have done three pregnancy test.the most recent one was two days ago,what do you recommend i do