I'm feeling a lot more tired than I usually have been. I'm worried I might have an illness of some kind, like chronic fatigue.
Thats not good, A doctor can usually diagnose you based on a your symptoms, You should get that checked out. 
What kinds of symptoms should I really be concerned about? Surely if I'm just tired I don't have to assume the worst, right?
i wouldnt assume the worst, But currently there is no known cure, But there is promising treatment for it.