I'm 19 years old and i am on the Depo Prova shot its a shot for birth control I've been on the shot for 1 1/2 years.. but I've  been watching my left nipple and it pulls in and it does all the time. it never stays out and i have discharge its not much and it never hurts but its a regular thing. my right nipple doesn't do this. any idea what  could be?