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Brown blood indicates that it is not fresh blood, but old blood. Are you sure you are not pregnant, because in pregnancy
sometimes brownish discharge is seen. If you are 100 % sure you are not pregnant, then stricture at the opening of cervix could prevent free flow of blood from uterus during menstruation
leading to some blood remaining in the uterine cavity
which oozes out later as brownish to dark brown discharge.
I think it will be a good idea to visit a gynecologist and get a thorough examination and ultrasound
study if indicated.