Since I have been reading a lot about birth injuries with my own complications, I have read about that a percentage (I can't remember how much, but it was at least around 30%) experiences birth injuries to the pelvic floor area including skeletal and muscular injuries showing up on a MRI scan. Since it isn't common practice to get an MRI done for new mothers it is often undiagnosed that a pubic bone may be fractured or the Levator ani muscle that often detaches from the pubic bone during delivery. Anyway, I am no expert in the field but what I know now is that the body takes a big knock with pregnancy and birth and that many women battle with years with all sorts of pain and discomforts and are often not treated propperly. Did you have the backpain from directly after the birth or did it only start later?
I had regular back ache after the birth of my first child (c-section) since I started picking her up and as she was gaining weight. I regularly had to see a physio therapist since my back ache was really intense. I was guilty of not picking her up correctly from the floor. I used to twist my body as I was picking her up, this is a big no-no!! My regualar back pain stopped when I stopped picking up my child when she was about 3 years old. For me, it was clear that it was from lifting her regularly and possibly not correctly as well.
Personally I would recommend that you see a doctor or physio that specialises in birth injuries that will be able to evaluate you.