Green’s Dictionary of Slang

(US) menstruation.

1999

J. Randall ‘A Visit from Aunt Rose’ in Verbatim Winter n.p.: Sometimes the visitors even have names — Dot, Dottie (incorporating the idea of dots of blood), Aunt Rose (incorporating the idea of redness, i.e., blood) or Aunt Flo (i.e., flow, incorporating the idea of the flow of blood).