What Causes A Missed Period With Negative Pregnancy Test?

I didn't had periods this month. The last it happened was on 20th January 2011, So the expected date in Feb was around 20 to 25 February.

I took a Urine Pregnancy Test Kit from the Medical shop and tested but it showed Negative.
Can it be possible that I am pregnant but the Test Kit showed Negative? Should I test once again? Or should I consult a doctor in person?