What Causes Nipple Discharge?

hi, Dr. im 23 years old & im unmerried girl, i have a ammbress problem, when i press my breast my nipples discharge a drop of liquid some times is look like a water transprent colour & some times its milky colour when this liquid is wet its sticky, is this liquid is harmful for me & when my boy friend suck my nipples its also discharge is this liquid is harmful for him? plz help me out of this problem im very confuse... thank you.