I had a touch of that back pain when I started too–I had a sore almost pinching feeling right at the lower back during backward bending. A teacher told me the pain was from too much compression in the lower vertebrae of my back, and recommended to focus on lifting my rib cage up and back instead of collapsing into the back bend. Basically I think I was “hinging” at the lower back with a mostly straight upper back, whereas she recommended creating the arched half moon shape with the whole spine equally. So now I think of trying to stretch up first and then go back like I am arching over a tall railing or fence by extending all my vertebrae fully upward and then leaning up and back. I can’t go back nearly as far using this form though, but at least my back stopped bothering me. Hope this helps you!
