If there is pain under the left breast when breathing, in addition to musculoskeletal sources on the left side, there may also be pleuritic chest pain caused by lung problems such as pneumonia, pulmonary embolism, chest trauma, immune system disorders. Acute pulmonary embolism caused by a blood clot from a venous thrombosis in the lower limb that has dislodged and caused a blockage of a blood vessel in the lung requires immediate medical attention as it is potentially life-threatening.