Please take your daughter to an Endocrinologist
for her physical examination and evaluation. Milk coming from a single breast can be due to high prolactin
level or high TSH levels but generally it happens from both the breast. Does your daughter also have irregularities in menstrual cycles? If yes, then please get her LH, FSH, Prolactin, Testosterone
and TSH levels done. Also one pelvis ultrasound
will help the Endocrinologist to decide on actions.
I hope you got the answer. get back for any further query.