What would cause intermittent pain under right breast towards the back radiating to right shoulder blade been going on for over a month?

It depends: There are many possible causes. An irritation of a nerve in the thoracic pain can cause pain to radiate around one side of the trunk. A gallbladder problem can cause pain radiating into the right side as can a lung and kidney problem. A detailed exam and appropriate studies can usually tell the cause. Consider being checked. Thank you for the question.