I have been having sore nipples for the past 2 wks, after my menstrual, but after taking two pregnancy tests they both say negative. what could it be?

Hormonal imbalance: You have some sort of hormonal imbalance possible between estrogen and progesteron because you did not ovulate or you had a chemical pregnancy. Take a blood test it will show a pregnancy possibly a chemical pregnancy too surely.

Hormonal response: Breast tissue, particularly the areola/nipple complex, is sensitive to hormonal fluctuations. More specifically, inflammatory changes, locally, can result in such symptoms. Along these lines, an anti-inflammatory medication like Motrin can sometimes be helpful.