Is it a good idea to have protein shakes (maximuscle etc) for chronic widespread pain?

I would doubt it: If you haven't see your doctor and see what the cause it. Some medications like cholesterol meds cause this. Lack of protein is not a big problem in the United States. Try a gluten free diet - although not great evidence some people report improvement for this type of symptom.

Not useful: This type of pain these days is referred as fibromyalgia. Exact cause remains elusive. Often patients have multiple joint tenderness as well as skin tenderness. Pain management specialist can help.