This condition is called galactorrhea
. Which means spontaneous flow of milk from the breast unassociated with pregnancy or lactation.
It is seen in 5% to 32% of the cases.
The causes are:
Hormonal imbalance like hyperprolactinemia
as in case of pituitary adenoma,
Hormonal imbalance caused by birth control pills also can cause,
Excessive nipple stimulation,
Side effect of some drugs like cimetidine, anti-psychotics like risperidone, proton-pump inhibitors etc.
As sometimes the underlying cause can be a serious disorder, you better visit either gynecologist or endocrinologist
to diagnose the cause.