Sever pain left side under left breast the er doctor said it us muscular but not real sure it really hurts sharp and stabbing pain ?

Follow up: Chest pain can be serious. If you are not comfortable with your diagnosis, by all means get a second opinion. You may want to discuss this with your primary doctor. Usually if the pain is reproducible, muscular origin is likely.

Agree with ER doc: I would agree with the emergency department physician, given the nature of the pain that you described. Breast cancer is unlikely; but if you are concerned, then please see your medical provider for an examination, testing if needed, and subsequent monitoring for any suspicious changes. Hope that helps.