Hi deblaforet and :welcome:
I'm so sorry to hear about your daughter...:hug:
As my little penguin has said, there are many EIM's of Crohn's and one of those are arthritic pain that can affect the large joints of the body as well as the smaller joints. One of the causes of lower back pain with Crohn's is sacroiliitis and inflammation of the sacroiliac joint. The sacroiliac joint is where the spine joins the pelvis. It has been suggested that 15-20% of those with Crohn's suffer with sacroiliitis therefore it is not an uncommon link between the two, shame the GP isn't more clued up about it.
Another thing to consider if this pain only presented after the commencement medication...Prednisone can cause bone pain and for some people that pain presents in the lower back and hips.
Also as has been said, Azathioprine normally takes at least 3 months to become therapeutic so there is some way to go yet until it fully kicks in. Hopefully the combination of Pred and Aza will start to pull things back into line and your daughter will find lasting peace and relief, bless her.
When do you see the specialist again?
Do you know what your daughter's inflammatory markers (CRP/ESR) are?
Dusty. xxx