Hi! I'm sorry you are having such a bad time. First of all, I have had temperatures with no illness. My doctor thinks it's fibro. I remember seeing him for a regular appointment and when they took my temperature it was 100 degrees. I take ibuprofen and Tylenol, to boot! But I felt just fine. So maybe that's what's going on but you are so concerned about it. It sounds like they have run a lot of tests on you so this could be a possibility. As I said, I take ibuprofen and Tylenol for pain and those are fever reducers and still, the lowest my temperature has been is 97 degrees. I just tell people I'm a Hot Mama! 
My son is ADHD and hasn't taken medicaton since he turned 17 years old. He was diagnosed when he was 3 years old and needed medication in order to function in school. He is now a very successful 33 year old man. I do still see the hyperactivity in him but that is good! He gets a lot accomplished in a day. But, your doctor might be right about the Dexedrine. I know you stated that you have taken it for years but sometimes, if we are on meds for a long time, they start to work differently. I know I'm on my third medication for diabetes because the others stopped working after a while. Dexedrine does have the ability to make an ADHD person tired, it seems to me. It is a stimulant and when you give a stimulant to a hyperactive person, for some reason it has the opposite results. My son was on Ritilin and it is a stimulant, too, and that's how it helped him. But, you have been on this a long time so it might be worth weaning yourself off it, like your doctor suggested, and see what happens. You obviously aren't happy with what is going on now.
Exercise really does give you energy. Right now you don't know how you could exercise, but you can, if you put your mind to it. I'm not saying to run to the gym. I'm saying, don't sit or lay too long, do some stretching exercises, and go for walks. I think you will find that you will have more energy then. The more energy you get, the more you will want to do, and the better you will feel. I know I do. I hate any form of exercise, but I started walking and feel so much better since! Some days I have to drag myself out the door but, once I get going, I start to feel better. Give it a try! It can't hurt you and it could help you.
Also, try not to eat a lot of carbohydrates and sugar. They can make you very tired, too.
Lastly, your doctor may be giving you really good advice but it wasn't what you were expecting to hear. I have a lot of respect for doctors that don't rush to write prescriptions for everything you have. His advice, to me, seems sound and it sure won't hurt you one bit to give it a try...a good, honest try...not just a day or two. I'd love to see you up and having a good time. You should! You are only 22 years old and this sure isn't fair, is it?
Also, you might ask your doctor about malic acid/magnesium supplements and vitamin D. There are links about these in the Fibro 101 thread...the first thread on the forum. These have really helped me with fatigue! They are finding out a lot about vitamin D deficiency and many with fibro are deficient in D and malic acid and both can cause pain and fatigue!
I do hope you get moving and getting some energy back. Feel better soon.
Sherrine