What Causes Severe Pain In The Joints And Muscles Of The Body?

My sister has been getting severe body pains on and off for several months. It hits out of nowhere, causing pain in her joints and muscles, especially in her legs. She has been to a rheumatologist who has ruled out any rheumatoid issues, and her blood counts are normal. She was diagnosed with low vitamin d and has been taking a prescription supplement, but the symptoms have not gone away and it has been more than 6 months since starting the supplement. The pains sound, to me, like pains one suffers with viral infections. Any ideas what else it could be? Her family doctor and rheumatologist are stumped.