I've had a serious issue with severe stabbing pain in my middle back to my left arm, have had heart and lung ruled out at er..possible torn muscle?

Latissimus: Very likely a muscle strain. Your "lats" do go from your back to your arm on the same side. A torn lats usually have sufficient soft tissue change to be picked up when you went to the er. Rest for a few wks and take anti-inflammatories +/- therapy. If not better by a month, see a doc.

Back pain: If its not heart or lung, it could be musculoskeletal (strained muscle), but it'd be worthwhile making sure its not gastroesophageal reflux disease as it often mimics heart problems.