At nearly a month after you had (I'm guessing, unprotected?) sex, you should definitely get a positive result from a pregnancy test if you are pregnant. You should probably go to your OBGYN to discuss what's going on as this is not normal for you. She can also give you a blood pregnancy test, which is a little more sensitive than a urine test. However, I am doubting pregnancy since it was nearly a month later that you tested.
you should go see ur doctor. it could really be ur period or it can b one of those weird periods where you are pregnant its called implantation bleeding. but if you take a test it'll come up negative for a few weeks. unless its a blood test