If you perform ghusl after menstruation before Fajr and pray 'Ishā' and Fajr, then find brown discharge after Fajr, knowing that this brown discharge usually prolongs your menstrual period, do you complete the fast?

Brown discharge (spotting) is considered menstruation if it occurs during the regular period or if it is continuous with blood. A ghusl (definition) (ritual bath) is required after its cessation, and any obligatory fasting days must be made up. However, if it appears after the end of the regular period, it is not considered menstruation and does not invalidate the fast.