The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
Because it is a herb specifically indicated to treat postpartum anemia as can be seen on Japanese Catnip's page.
Its main actions are: Relieves the Exterior and disperses Cold or Heat depending on the other herbs used. Releases the Exterior for measles. Stops bleeding. Abates swellings.