I recently got a doctor's diagnoses of Chronic Fatigue Syndrome, or CFS.
Sounds like a crap diagnosis, and in a lot of ways, it is. It's kind of a doctor's way of saying "I can tell there's something wrong, but I can't find anything in tests. But there's something wrong, and it's messing you up." Some may read this as the doctor brushing off a hypochondriac.
According to the CDC, however, it is a real condition.
What it acts like is Fibromyalgia's annoying, less-debilitating, little brother. But it does sometimes interfere with getting work graded on time.