What Causes Delayed Periods?

I am 20 years old. My LMP was April 18, 2010. Today is May 26, 2010. I have most pregnancy symptoms. I took a home pregnancy test which came out + on the morning of the 24th. On the 25th, I went to Dr., gave urine sample, got - test. I then got a blood test and got -. I still have not gotten period but still have the same symptoms. Please help!