Extreme fatigue and muscle pain

So for about two and a bit years now I've been really badly suffering with fatigue...not feeling tired or Needing to go to bed earlier (at the worst I was sleeping up to 18 hours a day and falling asleep at school) at first I blamed my anaemia, but according to my blood tests it's actually improved a lot from what it used to be. Now I'm not sure what to think. My symptoms are: fatigue, aching joints and muscles that get more and more painful to the point it hurts to move unless I sleep, weight gain and (though this may be unrelated I get pains in my lower left side of my abdomen...I was admitted to hospital or appendicitis because the pain meant I couldn't move and I had a fever of 103F. It wasn't my appendix. And it happens randomly for about 4-10 days at a time). Does anyone experience anything similar or know what this could be?