My breast tenderness, nipple sensitivity starts AFTER my period. That switched a couple of years ago for me and this is my typical pattern now. So, hormones can do this after your period, fyi. However, it's odd it is just one side, right? I know with breast cancer worry, we always worry about something happening to just one breast. However, this probably is not that. :>) I'd be more worried about infection of some sort. If it continues, I'd go ahead and have your doctor do an exam. I do think hormones can do this. but you want to make sure. No lumps? No nipple discharge, correct?