I got curious about the whole "I'm making milk, but I've never even had sex!" thing so I looked it up. Apparently its real and MOST of the time is caused by a non-cancerous tumor on the pituitary gland that regulates Prolactin. It can also be caused (though more rarely) by frequent mental and physical stimulation (ie: water rubbing her nipple as she swims all the time, her masturbating with her breasts).