What are the possible reason for a mid back muscle pain at left side? Not the backbone. It hurts when sitting on a chair and hand lifting.

Varies. There are a number of potential causes of back pain on one side rather than in the midline, these can include a facet joint, sacroiliac joint, or disc herniation problem; or potentially a back problem that has caused a unilateral spasm. Don't hesitate to be seen. A detailed exam and appropriate studies can usually determine the cause.