left is suddenly larger than the right and very firm...any thoughts?

Iam 25 and have noticed in the last couple of months my left breast is rapidly growing and not the right. I have felt both the right feels like regular fatty tissue but recently the left has become very firm and feels heavy at times. My nipple is VERY inverted now (never pokes out at all!) I am wondering what it could mean. It feels as if the left is made of 1/2 hardened clay. There are a few bright red dots that have appeared but I figured they're just freckles (They are nowhere else on me) if u could please help me. I did find a small lump back when I was 17 or so but mom said it was nothin & doc said not to worry about it then as my breasts were developing. They have not changed size/shape since I was 17 until now which is why I'm asking. I don't want to jump to conclusions so figured I see what all it could be. Thank you in advance for any help!!! Also no pain just some discomfort. Only pain is when I run or something like that they get sore.