Sharp pain on left side just under the shoulder blade / rib cage but only while taking a deep breath ? causing a lot of discomfort .

Be seen: Multiple potential causes, I would not hesitate to be seen. Potential causes could include pleurisy, pneumonia, thoracic radiculopathy, pancreatitis, or even PE. Again, don't hesitate to be seen, a detailed exam and appropriate studies can usually determine the cause.

Costochondritis: Sounds like costochondritis, an inflammation of the tissue in the rib cage. Usually follows mild trauma , frequent cough or strenous exercise. Icing the site and OTC meds may help. But if not improved after 2-3 days, please schedule a doctors appointment for a thorough exam. Best wishes.