My left arm is so achy?

Some time you may. Use it as pillow without knowing at night that to little pressure on the nerve and cause to be achy, hopefully it will go away. Thanks.
Arm pain. Arm pain may be a muscle-skeletal condition (strain, contusion, tendontis, bursitis) or a neurological condition (carpal tunnel syndrome, ulnar tunnel syndrome, thoracic outlet syndrome or cervical spine myelopathy='pinched nerves') if symptoms persist it is best to get seen by your provider. Rarely is arm pain a 'circulation' problem.
Be seen. Multiple potential causes. I would not hesitate to be seen for a specific diagnosis.