Hi,
I don't think that this pimple on your nipple has anything to do with breast cancer. The nipple contains glands and similar structures just like the skin of the face. If the pores get clogged and secretions accumulate, it can become infected especially if you continue squeezing it. Use antibacterial soap and keep it clean. It will probably go away on its own if you leave it alone. However, if you notice that it's getting larger, red and causing you pain, you should have it examined by your doctor or a dermatologist.
Take care