What is wrong with my breast?! Girls, please help!?

I'm a 15 year old girl. I got my period when I was 12. I'm currently on my period and have been experiencing a lot of pain in my left breast. My left breast is smaller than my right (I think by about half a cup size, but doctors say it's nothing of concern and it should catch up to my right). When I was 13 I got hit by a baseball in my left breast as well and the growth seemed to be stunted after that. I'm having a lot of pain in my left breast (about 4 inches away from my nipple, kind of where cleavage starts, sorry I don't know how to describe this haha) where my breast bones are. When I move around or turn my body to the left, the pain is almost intolerable. When I breathe or sit still, I feel pain but not as bad as when I move. I really want to cry. My mom thinks it's just growing and catching up to the right side but I'm concerned. I don't remember having pain this bad when I went through puberty. What do you think this is? Sorry if I was bad at describing this.