Dear back pain after delivery, happens mainly due to the physical and harmonal changes happens during pregnancy, it is normal, but it is just a temporary phase, follow these steps it might be helpful for you...
Make a practice of sitting comfortable, you can place pillow behind your back will help a lot
You can do oil massage weekly twice, or you can apply any pain relieving ointment, and take warm water bath, it will be helpful
You can feed your baby in comfortable position keeping an pillow behind, sometime this may also cause pain
If you gained more weight or your overweight, try to concentrate on diet, and start loosing your weight
Have an healthy diet with lot of fruits and veggies, will help a lot
You can also consult doctor or physiotherapist, they will guide you some exercise for reducing the pain