Have a on/off stabbing pain under left armpit.had a ecg in dec and all was good.dont think it's muscular so am worried ?

Stabbing pains are: rarely cardiac. They are usually musculoskeletal or neurologic. Cardiac pain is usually dull and pressure like. Squeezing and associated with other symptoms like shortness of breath. Ask your health care provider for a more definitive test. EKG is good acutely but perhaps a CT with calcium score or stress testing would be more reassuring to you.