Usually these lumps fall into one of two categories for girls your age- benign masses called fibroadenomas, or hormonal changes. But honestly, the best thing that you can do for yourself both medically, and emotionally, is get yourself looked at by your doctor.
Most breast lumps are benign, and this means that they are not cancerous. Breast lumps can also be caused by medicines that one is taking, breast infection or injury. When you notice a breast lump, the best thing to do would be to notify your doctor right away for checkup. Check out http://www.webmd.com/breast-cancer/benign-breast-lumps.
uh NO? Go to your doctor and have them look at it! It could be cancerous? and it could be nothing but if you find a painful lump anywhere on your body and you haven't done anything that you could think of that would explain it like an injury then you should ask your doctor about it! Especially if it is growing at all.
When I was about 9 years old when I began developing, I had budding lumps in both breasts, just behind the nipples. Yes they were sensitive , but they and the pain went away with time. This may be what you're experiencing if you're starting to develop.
I would certainly go to a doctor and have it checked. The most likely answer is that it's a cyst. When I was cycling, it used to happen to me all the time. They would appear then after a time disappear. Still, you should at least get it checked out.