It is best to first consult with your doctor who will examine you and give you the exact cause. Breast cancer is mainly cancer that forms in the cells of the breasts and there are numerous types of breast cancer. Find out more details here http://www.mayoclinic.com/health/breast-cancer/DS00328.
Is it a lump? Have you recently had breast augmentation? If it is hard and lumpy with no breast augmentation have a dr check for cancer. If you've had augmentation, it could simply be scar tissue build up or your implants still settling in. Ask your dr to be sure :)
If you are a teen still going through puberty then lumps are normal because your boobs are still growing, but if you are done with puberty then maybe you should get it checked out by your doctor. Hope I helped(: