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.
Japanese Catnip is a Neutral herb that tastes Pungent. It targets the Liver and the Lung.
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.
Abnormal uterine bleeding Abcesses Swellings Rashes Measles Rubella Sores Ulcers Metrorrhagia Metrostaxis