If you could feel the lump then whether or not chest x-ray reveal anything you need to have a biopsy
from this lump.
, a bone scan along with a mammogram
should be done to keep you mind at rest. These can sometime reveal problem even if normal X-ray is clear.
If the lump is just a feeling with no obvious swelling when doctor examines you, it could be a crack rib following violent cough. You are 51 and may have some degree of bone thinning (osteopenia
Hope this information is useful