I had heart cath last July and it was fine. I still get pain in mid back, sides and chest. I take Protonix (Pantoprazole). If not heart or GERD could it be stress? If not, that then what?

There are multiple reasons for your pain and you have either ruled out or are treating two of the most concerning. Another cause of pain you are describing is musculoskeletal, so follow with your primary physician for further evaluation.