Green’s Dictionary of Slang

(US) menstruation.

1919		T. Dreiser in Riggio Diaries (1982) 289: Helen is sick – monthly dues.
c.1950		in E. Cray Erotic Muse (1992) 369: Promiscuous girls live under a strain, / Waiting for their monthly pain.
1989		P. Munro Sl. U.