The pain could be due to pinched C7 or T1 spinal nerve. It can also be due to muscle pull. Apply ice, avoid straining your neck and left shoulder, take non-steroidal anti-inflammatory such as Tylenol, Aleve or Mobic, and if the pain persists, consult a doctor. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.