Weakness in my left arm (from shoulder to fingers). i work at a computer all day. ?

Tendinitis/neuritis?: If you are using your left hand for typing all the time, you may suffer from inflammation of the muscles, their tendons ( bone attachments) or the peripheral nerves. Either one of these tissues if affected may cause pain or weakness. If not relieved by cessation of the offending activity or within a few weeks, consult a neurologist.