What Causes Sore Nipples With Leakage Of Milk From Them?

I am 38....my breasts have been feeling sore around the nipples and my left nipple seems to release colostrum when pressure is applies. I already had three childre and can't have any more due to a tubal ligation and a uteral ablation. So why is this happeninging. Could is be due to nipple stimulation /sucking that occures during sex ?