What causes inverted nipples?

Multiple causes: Inversion of nipple can happen physiologically. In this situation you should be able to evert it by manipulation. If inversion is more fixed and constant but long atanding, it may be due to previous episodes of infection or mastitis. If it is more recent and progressive there is cause for concern. Since it may be due to an underlying malignancy. So, if you notice a new change in your nipple such as inversion, irritation or discharge you definitely need to have a medical evaluation.

Acute or chronic: The most common cause is congenital, meaning you were born with inversion. If the inversion is 'acute' which means, new, you should be evaluated by a specialist. You should have a mammogram +/- ultrasound to r/o a mass/growth.