Woke to ache in left breast/chest. Shoulder blade pain later. Then stabbing pain from shoulderblade through to breast. Muscular? Nerve? Cardiac? Fear of mi

Many causes. It is not possible to determine the cause of your pain based solely on your description of its location.
Pain. The pain description sounds musculoskeletal. If you have cardiac high anxiety, you should have a discussion with your doc about your cardiac risk factors and do your best to minimize them to minimize your risk.