See your entry on my "Frustrated" post and do the same
Anything set you off recently? Co-worker? Family? Overload on activities? Any kind of stress even in small doses builds up to exhaustion.
Sometimes your body just says, "I've had enough for now" or "I'm bored" and goes on a mini vacation. Rest up. If you aren't feeling better in another week or so and it's unusual for you, see a doc.
Change anything? Medication, foods, diet? If you are into suppliments, make sure everything works together, specially if you take meds. Don't ask how I know.
Let us know how you are doing.