It could be anything from cancer, to clogged ducts, to infection, to fibrocystic breast disease (which is benign) to other chest ailments. You should see your doctor.
It could potentially be any number of problems: 1. Angina 2. Reflux 3. Musculoskeletal pain (muscle strain) 4. Breast pain 5. Chest infection To get a proper diagnosis you would need to be more...