(adjective) When a woman is "on the rag", that means she has her period. She's also likely in a bad mood because of the various discomforts associated with pads, leaks, tampons, washing out underwear, not being able to wear light-coloured skirts or pants, etc.
She's really in a mood, she's on the rag and complaining about everything. Last night she slept so soundly that she woke up in a pool of blood.
By Zea
On The Rag
in older days, ladies used washable cloths (rectangular pieces) in the gusset of their underwear during menstruation or to prevent unforeseen pee discharge, the term was made synonymous to a menstruating lady having moods & quirks .