Are brown discharges after a D&C considered menstruation, which would prevent fasting and prayer?

We do not know the duration for which you saw blood and the brownish discharge mixed with blood. If the total number of days of blood, followed by blood mixed with brownish discharge, and connected to it, brownish discharge, does not exceed fifteen days, then all of it is menstruation. You must wait for purity, indicated by one of its two signs: dryness or the white discharge. If the total duration exceeds fifteen days, then you are a woman experiencing irregular bleeding (istihada). In this case, you should refer to your previous habit. If you do not have a previous habit, then act according to a valid distinction (between menstrual blood and other blood). If you cannot distinguish the characteristics of menstrual blood, then consider six or seven days of the month, by estimation, as menstruation, and anything beyond that is irregular bleeding.