Hello, I have gone through your query and understood the concern. If you have always had regular cycles and never any delay, are sexually active without any protection, your chances of conception are high. The brown discharge you had during your previous menstrual period is suggestive of some residual/ breakthrough bleeding. Hormonal imbalance also is a possibility. In the current scenario, please wait for a week and if you do not see your period, go for a pregnancy test and see a specialist for further advice. A trans-vaginal sonogram at that stage will help to clarify. Hope this helps.