(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.
af Jake 20. februar 2004