Any ideas why is it that I have pain in my left shoulder, that shoots down my arm making my thumb hurt really bad?

TOS. Absent neck pain, shoulder pain ; associated arm parasthesias suggest thoracic outlet syndrome (tos). Diagnosis is made by ultrasound evaluation of ipsilateral subclavian artery with addson's maneuver. Osteopath can diagnose. Have you had shoulder injury? Have you had chronic low back pain from mechanical injury; leading to shoulder asymmetry ; pain ; from spinal scoliosis due to sij instability?