Why do my boobs look like this?

The past few days they have been hurting so much. I have missed my period so don't say 'oh you're getting your period!!!' I'm 12 years old. My left boob is red around the nipple but not on the right. On my right boob is totally normal. And the left boob the nipple is flat. I haven't been jumping or anything like that. It is also very hard when I feel it on the left boob. What's wrong with my boobs!?! Am I only having puberty on ONE SIDE??!!