Our bodies have to adjust to our “new” posture and way of moving. I had back pain at one point in my recovery, when I started walking for longer distances. My body was so used to my old, broken down knees and gait that my new correctly aligned knees caused the muscles in my hips and back to protest. It took weeks to work it out. I just kept walking as best I could, practicing good gait and posture, and my body got back into good alignment.
It it’s any consolation, I have fewer back problems now! Haven’t had back spasms in several months, and I used to have them all the time. So the overall effect has been good.