I say, and with God's success: She must perform the ritual purification (ghusl) after sexual impurity (janabah); because janabah is a continuous matter, so she would be in a state of janabah after converting to Islam. This is unlike the case if her menstruation stopped while she was a disbeliever and then she converted to Islam, as she would not be required to perform the ghusl; because at the time of the cessation, she was a disbeliever, and she is not commanded to follow the religious laws according to us. Whenever she converts to Islam, the cause, which is the cessation, no longer exists; because the cessation is not continuous. Refer to: Sharh Al-Wiqayah, p. 95, and God knows best.