What if it’s pregnancy?
For 20-30% of women, pregnancy can start with the so-called implantation bleeding (IB), which can be similar to brown discharge instead of period. To dispel your doubts, you should keep in mind that IB occurs a few days earlier than regular periods and is not accompanied by cramps. Besides, implantation can be followed by other pregnancy symptoms, such as sickness, taste disturbance, etc.
One time brown mucous discharge instead of period, especially if it was preceded by one of the mentioned factors, is not a cause for concern. Reoccurrence of such a phenomenon must put you on alarm and prompt you to visit a clinic, where etiology will be accurately established and corrected to the extent possible.