Have you had kind of like a stomach flu or any kind of viral symtom illness for some time directly preceeding all those symptoms? Are you noticing a pale or light-yellow discoloration of your stool? Sometimes following or accompanying a chronic viral infection, small degree of malabsorbtion (inability of the intestine to absorb vitamins/nutrientsor fats) can develop, and be associated with weight loss, loss of appetite, flushed cheeks, very low grade fevers, lets see... dry, itchy skin skin, some tingling in arms and feet, some nausea, and as far as blood counts, borderline normal platelet counts (150-160-170)(not anemic). And I'm not talking about one of those symptoms for everyone else that reads this message, but all of those symptoms present at the same time :).  Malabsorption usually results from loss of digestion related vitamins and minerals in the small intestine, so it can be helped significantly by taking multivitamine supplements, or much better, drinking multivitamin supplements (like Ensure), since body absorbs vitamins in liquid form easier.

    Another thing to look for that would not show up on the tests that you mentioned, but could be related to your symptoms is gastroparesis - some degree of disfunction of the stomach muscle, which does not allow the food leave your stomach appropriately. This kind of condition can be related to prior surgeries in the abdominal area, viral illness, diabetes or just be plain idiopatic (meaning that doctors really have no idea) This could be ruled out by doing a gastric emptying test, which consists of eating a radioactive-labeled meal, and then nice people will monitor the time it takes to leave your stomach. If this condition is identified, the doctor will probably prescribe a pro-kinetic like Propulsid, in the absence of a root cause that can be treated.

    Being sick for 8 months or more is certainly not fun, I can tell from personal experience, and I'm sure you're worried about it. Matter of fact, worry and stress contributes significantly to chronic sickness, so it would also help (seriously :)) to try to calm down. I know it's not easy, and you cannot just tell yourself to calm down, but instead try to take your mind off of this problem by doing stuff that is fun to you with other people. In any case, with all the high-power tests like blood tests, colonoscopy and upper GI series coming out normal, all  chances are that this is nothing really serious and could actually be Irritable Bowel Syndrome, which is diagnosed by ruling out everything else, which explains the multitude of symptoms it can be associated with. It is the ultimate excuse of all the Gastroenterologists out there.    

    Take care, God Bless.

I have had the same problems. just found out I have Hyperinsulinemia. Too much insulin-- not diabetes--.
Ask your doc to run some blood tests to check for it. Best of luck!!