ive got gyno, (waiting on the docs orders) and it started in my left nipple when i was about 16 then when to my right about 6 months later. also with his diet chicken is pumped full with environmental estrogens. stay away from it. also i used to drink about a gallon of milk every day or two. not really sure if that had anything to do with it, just sharing from my experience.
but....anyway.... go to a doctor immediatly, make a HUGE deal about it, tell him to get on the fucking ball and find out what it is. my doc said "ohhh thats just your hormones itll go away in a few years" yeahhh buuuuulllshit!! my doc didnt do this and now im 18 with fucked up gyno