What Causes Severe Muscle Twitches,lower Back Pain And Fatigue?

I am a 50 year old woman of normal weight/height and a no history of serious health problems. About a year ago, I started having the following problems and they have continued despite numerous visits to doctors, tests, etc. These are my symptoms: 1) severe muscle twitching all over my body, including stomach, arms, legs, back; 2) severe lower back pain which has been somewhat alleviated by physical therapy but by no means eliminated; 3) fatigue, as in, still exhausted after a good night s sleep - to the point that I have at times considered calling in sick simply because I was too tired to work -- but ultimately I went anyway; 4) memory problems --- I have become very forgetful; 5) cognitive problems, mainly in the form of inability to concentrate at work ( I am an HR admin for a commercial real estate company). This has been very troubling to say the least. I have been to neurologists, orthopedic doctors, and just about everyone but an MS specialist (I read an interview with Ann Romney and her description of the fatigue she suffers as an MS sufferer sounds very much like mine).