The most probable cause seems to be abnormal/low blood supply to your left arm while sitting.This blood supply gets normal while in standing or other positions.Other common causes of numbness are:nerve compression syndromes, pressure on the nerves,,diabetes, multiple sclerosis,stroke,transient ischemic attack.Also this can be due to :underactive thyroid,abnormal levels of calcium, potassium, or sodium in your body,lack of vitamin B12,certain medications.
The underlying cause of numbness or tingling should be identified and then treated by your doctor.Tests like electromyography and nerve conduction studies,certain blood tests,electro physiological studies are required to make the exact diagnosis.Doppler studies are required to see whether there is proper blood supply to the arm.
The exact diagnosis can only be made after clinical examination & tests.
So I advice you to make an appointment with your doctor for tests to know the exact cause & treat accordingly.
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