What Causes Delayed Periods?

my name is allyson i am from illinois and i am a mother too 3 beautiful children. Well i have unprotected sex and my last peirod started april 5th, 2011 and i still havent recieved my peirod which its saying i am 15 days late now and i have taken home pregnancy tests which have came back to be negative and i have been to the dr to do a urine sample which also came back negative i havent been too get a blood sample was just wondering what i should do?