I am on a Facebook chat site for Chronic Fatigue Syndrome and this is a common complaint in this group. I have read fibro and CFS are two different things. They do have some similar symptoms. Might want to look at CFS and also anemia. I have experienced weakness, fatigue and stiffness -which I could call "heavy muscles". Most notably, it prevents me from getting into the shower to start my day. One more observation; when I am having a flare with these symptoms and I go for a massage (the only thing that helps), the masseuse has observed that there is a type of swelling under my skin. I've wondered if that could be inflamed fascia.