Post left pneumonectomy pain in left arm, shoulder and hand. work on a computer 7hrs/day. what is causing numbness/pain/tingling in my left arm?

Could be from: The procedure( irritation of pinching of the nerve ) but also you may have unrelated cervical or lower thoracic degenerative disease that is causing the symptoms. I believe you need to see either a neurologist have and through evaluation and run certain test like nerve conduction test and possible MRI of the cervical and upper thoracic region . Clinically you have neuropathy.

Nerve impingement: You don't say why at 27 you had "pneumonectomy". Either it is a result of the underlying problem or a nerve could have be impinged upon or damaged by the procedure itself or simply by positioning during anesthesia. It can take a long time for damaged nerves to recover. Meanwhile they be distorting the information regarding pain, sensation, hot/cold and touch going to the brain and info to arm.