Is she on medication(s) could be side effects.
Has she had her nutrient levels checked? Especially electrolytes? How's her diet? Has she tried cutting out dairy, wheat, and other common allergens? Seen an orthopedic surgeon to see if there are any hip or back problems pinching on nerves? If she's tried absolutely everything, it's time to see better docs, such as going to Mayo Clinic or Johns Hopkins. But sometimes, when everything looks good and it isn't, something to try is getting away from doctors and trying natural medicine, which will focus on diet, etc. Sometimes that works when other things don't. I've seen it happen with people with hard to treat things like fibromyalgia, chronic yeast problems, etc. Sometimes doctors aren't the right people, but first you do have to make sure there's nothing there that docs can find and when your docs can't find it, find better docs.