Off/on squeezing in left shoulder, left arm pain, and sometimes brief, heartburn like pain in center of chest. in past, ekg was fine. what is wrong?

Chest pain: Of course, heart issues are the major concern. Less likely to be coronary artery disease which is the most feared in any case of chest pain given your young age. I believe you will need to see a doc, because more questions are to be asked and physical exam is a must.

Not your heart: Unless you are diabetic or use cocaine, this is not your heart. "common things happen commonly". You almost certainly have a strained left shoulder muscle and intermittent heart burn.