hi, thank-you for providing the brief history of you. A thorough examination of the chest and costochondral joint is advised. An x-ray or MRI will help much to understand. It looks more of a costochindritis and needs to avoid exercises for time being which aggrevates the inflammation of the costochondral joint. Also, doing breathing in that duration of rest while placing the hand on the painful area will help strengthen the muscles. As intercostal muscles are smaller and needs a thorough understanding and proper exercise a physical therapist should help better. In my clinical practice most cases respond well to physical therapy guidance. Regards Jay Indravadan Patel