Ashley, take a deeeep breath; and relax. Chances of you having breast cancer are way less than 1 in a million; our message to teens elaborates on that.
The pain and possible lump you're feeling are almost certainly hormonal, and entirely age-appropriate. During puberty, hormones become active and pass the message along to your breasts that it's time to grow; breast buds (small, hard areas of tissue) develop, and eventually develop into woman-sized breasts. While this happens, your breasts can feel sore and tender; this is normal.
It's hard when your physical development doesn't keep pace with that of your peers, I know; I was in the same situation as you. Very embarrassing, especially in gym class! But eventually, at our own pace, we all grow breasts. Large, small, or in between, there's no telling for sure their final size; but check out pictures of your mom when she was young - there's a good chance your breasts will resemble hers, size-wise.
And please don't be afraid to talk to your mom about this; she'll understand more than you think, both physically, and emotionally. Remember, she went through puberty, too; and it doesn't change all that much, generation to generation. Best she should know what's going on with your health; and when you're worried, sharing with someone else is really, really helpful. So talk to her, OK? I think you'll feel much better. Take care - PJH