Mystery Back Pain?

My GF (25 5'2" amputee left leg) has had ongoing back pain in the lower area. I would say it's where the spine meets the hips. There are times where is bothering her really bad. One time I tried helping her up and she just couldn't do it. She raised up and fell back down crying because of the pain. Doctor's have done an X-ray and said nothing is wrong and they left it at that. So basically no help, my question is could it.be an ill fitting prostectic? She is overweight could it be contributing? She gets constipated every so often don't know if thats useful information.