What would cause pain under your left shoulder blade?

Hello. Thank you for posting.

The most common cause of shoulder blade pain is a muscle strain. Short-term overuse of your arms and upper torso may be experienced in your scapula. Other muscular conditions which may cause shoulder blade pain include rotator cuff tears and a condition known as snapping scapula syndrome.   More information below:

https://www.healthline.com/symptom/shoulder-pain 

Helena