What Causes Persistent Back Pain?

My niece is only 34, she has been having back pain and was discovered she has two compressed disks, has very brittle bones as a 90 yr old. Was told not to lift more than 5 lbs as she can break one that easy. She is in constant pain, just had her 3rd child 5 mo. ago and is still nursing. No one has been able to give her any answers. They are sending her to Brigham and Williams in Boston for more testing, but have not informed her what test they are considering. It is at the point that she can't even touch a rib without pain.
Do you have any idea what could cause such pain in someone so young?