Bilateral Muscle Pain in Arms, Legs, Neck and Shoulders

For the past 6 months I have been suffering from severe bilateral pain and stiffness in my arms, legs, neck and shoulders. I have been to several doctors (2-rheumatologists, Neurologist, endocrinologist, family doctor) and they have done multiple blood tests, some CT scans and an ultrasound of my thyroid, physical examinations etc. The best way I can describe the pain is that it feels like my body was injected with lactic acid (like I did a full body workout without ever having worked out before). It puts me in tears just to take a shower or get dressed. My muscles do not hurt when I touch them, only when I flex them or use them. I do not have joint pain. When it first started 6 months ago it was extreme pain in my upper arms and legs and has since spread to the other areas. My abdomen and back muscles do not seem to be affected. They have ruled out Polymyalgia Rheumatica, lupus and many other things, but cannot figure out what it is. Please help!