there can be some obstruction to blood flow of your left arm .get colour doppler for blood flow & find the cause of obstruction,it can Thoracic outlet syndrome
which is a syndrome involving compression at the superior thoracic outlet involving compression of a neurovascular bundle passing between the anterior scalene and middle scalene. It can affect the brachial plexus
(nerves that pass into the arms from the neck), and/or the subclavian artery
or vein (blood vessels that pass between the chest and upper extremity).
The compression may be positional (caused by movement of the clavicle
(collarbone) and shoulder girdle on arm movement) or static (caused by abnormalities or enlargement of the various muscles surrounding the arteries, veins, and brachial plexus).