What causes your breast to discharge a clear/milky liquid if your not pregnant?

Nipple discharge: Could be secondary to increase activity of the pituitary gland, hypothyroidism, pituitary adenoma(tumor of the gland), medications, chronic stimulation of the nipples, breast cancer. In any event this needs to be evaluated by physical exam and lab tests.

Pituitary, meds: The two most common problems for discharge from both breasts is medications (antidepressants, antipsychotics, etc) and pituitary problems (check the prolactin blood level). If only from one breast, sometimes this can be from a cyst, abscess, etc.

Prolactin or not: Either a rise in a hormone called prolactin from the pituitary gland due to many reasons or idiopathic (no reason can be found) causes are tumor , inflammation , hypothyroidis , medication , chest wall trauma etc.