If I got a painful breast lump at the age of 15, could this be breast cancer?

No. A breast mass at this young age is usually a fluid filled cyst or a noncancerous mass called a fibroadenoma. I recommend seeing a physician or gynecologist for a physical examination.
Not likely. Most breast lumps in 15 year olds (men or women) are either part of development or the result of injury. But any lump that concerns you should always be checked out by your doctor.
Rarely. Brest cancer in teens is very rare. This should be examined by a physician so you can know without a doubt that you are safe.