What Causes Unilateral Spontaneous Nipple Discharge?

I am a 39 year old female who has had unilateral spontaneous nipple discharge. Because of this I have had a mammogram and ultrasound and both have been normal. I have been examined by 4 doctors, due to my anxiety over this, and continue to worry about the discharge, though they don't seem concerned. This started 1 year ago and sometimes it happens once a week, and other times it doesn't happen for months in between. Any ideas?