What Causes Chronic Breast Tenderness And Discharge?

I am a 19 year old female, healthy as a horse. However, for the past year or so, I have been experiencing nipple discharge. Most of the time it's unilateral and from the left breast. It only occurs when I am laying on my side or if the breast is squeezed. I have had a brief breast examination from my family doctor followed by a CBC to rule out any possible causes for the discharge. My prolactin levels came back elevated, so my family doctor suggested I got an MRI to rule out prolatinomas. It has been a little over two months since my MRI came back clean and I am still experiencing some localized, non-cyclic tenderness of my left breast and discharge.