Unexplained chest pain what could it be have had it for 3 months ruled out heart and gi problems and back of left arm could it be muscles?

Chest pain: Probable radicular discomfort due to nerve root irritation-go see a neurologist to assess

R/o pulmonary issues: Given back of arm involvement, neurological cause is most likely, but would at least get an x ray to make sure there is no pleural effusion. Otherwise most likely musculoskeletal. I am assuming that you're otherwise healthy and active.