on the rag

From rag as a slang term for a menstrual pad. First use appears c. 1967. See cite below.

• "She herself had begun menstruation shortly after her eleventh birthday and had gone to the head of the stairs to yell down excitedly: "Hey, Mum, I'm on the rag!""
• "Harold: How is Sarah? I don't want to jump her while she's on the rag."
• "On the rag, in a bad mood.—College males, Arizona. I had a lousy time last night because she was on the rag."

• on one's period