Breast changes

So the last 4 days my breasts and nipples have been super painful, like to the point I can't even wear a shirt and today I noticed that my nipples have started turning brown. The only thing is, I am only maybe 2-3 days past ovulation according to my opk (waiting for one more bbt to confirm) so obviously I'm not pregnant as this all started a day or so before I even ovulated. Does anyone have any idea what it could be? Should I be worried?