Hi there, mostly postpartum back pain is due to weakness in back muscles, breast feeding which is a must in seating position. You need really nutritious food, along with iron ,
calcium and Vit D supplements.
Physiotherapy for strengthening back muscles along with this will really help to reduce your backache.
I do not recommend uses of belt for back pain or loose abdominal muscles. Start household activities and half an hour gentle exercises. You can consult online for further advise.