What Causes Frequent Nipple Discharge Mostly During Periods?

i have a milky discharge coming out of both nipples has been happening for 14 years. it occurs more when i'm ovlating and while having my period. the only time that it doesn't happen is the first few days after my period has went off. why is this one more question. the bottom of my uterus is hard and has been this way since i had my first child. i heard that this should only happen when you are pregnant. no abnormal pap test