There could be many reasons for such a condition. However, the most probable condition that I can assume by the conditions you have stated is pregnancy. If you period was pinkish brown in color, then it could be implantation bleeding. It is the bleeding that a woman experiences when she conceives and it is around the time of her period.
Another reason could be a miscarriage. It is possible that you got pregnant without you knowing and then it got miscarried in very early stages. However, whatever might be the case, do consult a doctor for it.
