If you were sent for CT scan they were probably worried about a pulmonary embolism, but instead found a pleural effusion which commonly causes the kind of pain you were experiencing, probably on the base of the left lung and irritating the diaphragm which can radiate to the shoulder.
Thanks The pain was so severe that I was sent for a CT of the chest on Fri pm
Still so bad on Sat back to ER and admitted.. another CT with contrast showed a pleural effusion I can not believe the pain So glad I had an IV and narcotics! My girls kicked it in 24 hours but my Doc said in an adult it can be severe. Especially hard to diagnose as no fever or symptoms that would send you to the doc. Now I understand more about the nerve that goes from under ribcage to the shoulder helped explain how the pain traveled and caused me to breath so shallow
Stay well.
That sounds too much in 1 place to sound as much like influenza which usually makes you sore all over with fever and chills and runny nose and cough. Sounds like you and your 12 year old may have a different virus.