All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The brown fluid she experienced known as the ‘brownish discharge’ is considered menses only if it flowed from her at the time of her usual menstruation. In case it is considered as menses and she was unsure as to whether it flowed from her before or after sunset, then her fast is valid because menstruation is considered to exist only at the time when she first became aware of it, for the general rule is attributing an incident (menses in our case) to the most recent time (after sunset in our case) and not earlier than that.
Allaah Knows best.