What is the Pain my Mother's Feeling?

My mother is 58 years old, years past menopausal stage, and at least a couple of times a week, she gets these widespread pains, first in the back of her head, then across both , and even on her knees. She's been to many doctors, none of whom have found anything wrong. She had an MRI on her head--no problems. They said it's not her heart, not cancer, not anything; and her symptoms are not consistent with fibromyalgia from what I've researched. Has anyone experienced anything like this or known someone who has, OR has any idea what is going on? I would greatly appreciate any help.