This pain in your left shoulder blade has been going on for along time , this might be comforting as it does not sound as if the pain has got worse over time. As the MRI
was apparently normal one should also remember that you can also get a referred pain
from the spleen
and diaphragm that presents in the left shoulder pain
. It is best that your doctor examines you again and monitors any change over time. It does not sound as though the " lump" you can feel necessarily has anything to do with ongoing pain, although the lump might be painfull on its own. Perhaps your doctor should consider to have it removed.