I believe 2nd delivery is also by c section.
Pain after cesarean section is usually seen in most of the patients sometime it's because of the pregnancy itself or sometime it's because of the spinal anaesthesia
Next Steps
it is not advisable to give too many medication for back pain because mother will be lactating
Health Tips
take care of the back by maintaining correct posture of the back you can do hot fomentation over lower back you can even use Pelvic binder or lumbosacral corset