What Causes Delayed Periods?

Hi I am 26 years old My period is delayed by 4 days expected to had it on 4 June 2011 but still there are no signs i had sex with my Bf just a day before the expected date of my period .This was the very first time we had sex is that my period is delayed that i am pregnant please advice There is no discharge in my vagina and is dry i am nervous please help