What can cause back pain by the ribs on left side?

Movement?: If worse with movement likely a musculoskeletal injury. If not, could be any one of a number of things. See your doc, go over the details and get examined.

Need to examine: It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. If you did not hurt the area, it is likely a muscle sprain. You may take acetaminophen, not more than 4 g/day, no alcohol, do not limit your activities. Exercise as tolerated and gradually increase the time and intensity of exercise. If it is gets worse, see doctor.