Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) said: “If this brownish discharge is the precursor of the period, then it is hayd (menstrual bleeding). You know that it marks the onset of the period if it is accompanied with pains and cramps. But if the brownish discharge comes after the period, the woman should wait until it goes away, because the brownish discharge that follows on from the period is also hayd, as ‘Aa’ishah (may Allah be pleased with her) said: “Do not be in a hurry until you see the white discharge.”
(Risaalat al-Dima’ al-Tabee’iyyah, 59).
Based on this, if you think that this brownish discharge is the precursor of your period, then it is hayd, so you have to stop praying and fasting, and make up your fasts after your period ends.