According to Georgia's GlossaryEdit
An expression to indicate that a girl is... er... having her... you know whats. Oh come on, you do know. Having her... er... well to put it plainly... her... we that the "red flag is flying," that her "little friend has come to visit". Period. Menstruation. Menses. Women trouble. Trouble at the mill. I can't go on with this; it is making me tired.