I just wanted to post this in case it helps anyone suffering from postpartum back pain. I had awful back pain for a couple of months from about 2 months postpartum. It started in my rib area but extended to my mid and lower back. It was painful to lift and care for my baby, uncomfortable to lie on at night, and I'd get awful spasms when I got out of bed.
I saw two physios and a GP, all who said I'd pulled/torn muscles and need to strengthen my back with exercises. One of the physio's exercises made the pain a lot worse. I was starting to despair until one day I happened to put on a non-nursing bra I'd had pre-pregnancy - and my back felt instantly looser. I realised that the nursing bras I had hadn't been providing enough support when I was full of milk from a hungry baby! Since changing bras my back has to gradually improved and I'm back being able to do most things I want to. I realise this won't apply to everyone with postpartum back pain, but I wanted to share in case anyone finds themself in the same situation.