Could the shape of my back cause severe back pain, especially after an accident, and after treatment from a chiropractor?

More than likely . There are changes in the appearance related to the pain from your injury. A loss of the sway in the neck and/or back is a result of muscle pull/ tension/spasm not a cause. There are also deformities that can affect spine appearance and can also at a role in pain like kyphosis and spondylolisthesis. See a spine specialist if not improving .