Hi!
Avalik is correct, the usual cause of lactation is an elevated prolactin. The usual cause of elevated prolactin is either a pituitary adenoma (a growth on the pituitary gland) OR it can be a side effect of a medication. Often there are other symptoms such as infertility or irregular periods. Most of the time the first thing to check is a prolactin level, and then if elevated, an MRI of the pituitary. Dostinex or Parlodel are often used to treat this condition.
Hope this helps!
Dr B
Oh, you know, I had the same problem starting at 16.
It was because my prolactin levels were too high ... caused by a medication (Risperdal) I was taking. The doctors had me have an MRI in case there was a tumor on my pituitary gland, but no it was just the medication.
So ... go to the doctor, tell him/her the issue s/he'll draw some blood I assume and see where it goes from there.