You have no idea how often contortion coaches get asked for lower back drills followed by a comment saying how tight their lower backs are due to the struggle they encounter whilst bending backwards.
Majority of the time when the lower back hurts it doesn't mean that you'll need to stretch it more. There are various factors that impact your painful lower back - tightness of your upper back and hips to weakness of your glutes and core.
The body then compensates and loads up the lower back more than it should - hence, causing you the grief.
Although back bending looks easy-breezy to most people, a healthy back bend is a little more complex and actually requires a fair amount of strength. SURPRISE!
So if you're feeling all the pain accumulated on your lower back, I highly suggest for you to step back and take a closer look.
However, if your lower back is consistently giving you trouble and worsening over time, please seek professional help like an osteopath or a physiotherapist.