I think that should be the next step if the "usual suspects" in pain relief aren't working and the back isn't getting better. If its thought that treatment could help her back then an orthopod and if its chronic and intractable then a pain specialist consultant or nurse. With respect to practice nurses, don't let her be fobbed off with the "practice nurse who has an interest"
Yu are probably aware that constipation deffo makes back pain worse and its really hard to go to the loo at the best of times with back pain, so I would suggest that there is some urgency to getting this sorted.