Im 20 years old girl.5feet or 149cm(myb) n my weight is 49, and since 1mon ago,my waist on the left back always pain after laying on the bed.I cnt bear it anymore. Last tme,after I cheacked hspital,my x-ray result is OK and there's nothing wrong around my waist n my bone is good. Until now,I still don't know why my waist on my left at the back still in pain n its hard for me to walk in normal way.(like old-women) hehe.. sory if my sentences are funny cos I'm not very good in english.

No one can make an accurate diagnosis on line and your symptoms could be any number of things,including the way you sleep.
I would revisit the doctor or get a second opinion.