Which kind of test was negative? If it was a blood test, then you probably aren't pregnant. They are almost 100% accurate. You may have had what is called a "chemical pregnancy", which is a very early miscarriage. (http://www.ourmiscarriage.com/chemical_pregnancy.html)
Of course, there is always a possibility of a mistake, so ask your doctor to test again just to be sure. I wish you the best of luck.