lump in breast

So when I was about 8 yr/o i noticed a small lump starting to form in my right breast and soon one started in the left one too. My grandmother was recently diagnosed with cancer at the time and I was terrified that thats what it was so i didnt tell anyone. Now im 15, still havent told anyone and both lumps are around 3 inches wide. I was wondering if anyone had an idea of what they might be. they have always been centered in my breasts and can move around a bit but they are fairly hard and painful. I was thinking of telling someone soon but I just wanted some idea of what it may be before I say anything. Thank you So Much!