What Causes Delay In Periods?

hi, my periods last was on april 1st 2011, and is expected on 1st or 2nd of may 2011, i dont it was in the last week of april that i had pre-ejaculation sex with my husband i am a 32 year old working lady with a kid of 6 years old....i am 34 kgs of weigh i know i am underweight but hyper active and healthy...i do feel tired and weak sometimes...but am more tensed as to why it is delayed...it has been beofre delayed by max two weeks..please advise...am not ready for a kid right now...