In the areola around the nipple, women have Montgomery's glands and sebaceous glands that can get blocked and swollen.  Of course, you can also have the same kinds of pimples, warts, or moles that occur anywhere else on the body.  I don't know exactly what might be causing your bump, but it's almost certainly not breast cancer.  A good test of how concerned you need to be is to think about what you would do if the bump was on your tummy or arm.  If it is a wart or infected gland, you probably will need to see a doctor.  If it hasn't gone away on its own in a week or ten days, give the doctor a call.  If it gets red and inflamed, call the doctor sooner, especially if you run a fever.