Given the data you provided (pain under right breast, medical history of lung cancer in 2002, negative x-ray and negative mammogram) you might have felt scar tissue that formed after the surgery. It is not uncommon that the damaged tissue (as in surgery) produces fluid which is mainly formed of lymph. Within several days the fluid is absorbed, but sometimes the solid part of the dissolved materials in the fluid might stay and make the body form a connective tissue around it. This is presented as a scar. Depending where the connective tissue is located, it might cause pain when it pulls the organs it is attached to. For example, if the connective tissue is between the two layers of the pleura, you might feel pain when you inhale or exhale only.
You might want to visit a thoracic surgeon for more information on this issue.
Please note that the location of the pain is very important. If the pain is below the right breast and even lower (below the right ribcage), you might have experienced gallstones, for which you might consider visiting a specialist for gastrointestinal disorders.