What Causes Delay In Menstruation Other Than Pregnancy?

I am 21 years old, sexually active, and am having issues with my period. I have been on birth control for a year, and my last period began on December 6, 2009 and lasted until December 11. Since then I saw my gynecologist and told him that I hadn't had my period but was still on the pill, and he ran some tests and said everything looked fine. This month I had some spotting - pink one night, brown the next day, and then nothing. I took a pregnancy test a week ago and it was negative. Any clues as to what could be wrong with me?