Always Tired...why?

Why am I always tired? I get plenty of sleep each night, and I sleep soundly too! But every day, at least for the past week or so, I am exhausted by 2 p.m. and seriously just crash on the couch. We're not talking "taking a little nap." No, we're talking lay down, eyes heavy, headachy, and down for the count. Two or three hours later, I open my eyes but feel like I've been drugged. I can hardly get up from the couch. I continue to feel druggy the rest of the evening, so I go to bed, and start the whole process again the next day. I'm baffled.

The bad thing is that I'm supposed to start the AP Reading this week. That means 8 hours a day sitting in hard chair at a big folding table scoring student essays. How am I supposed to stay awake for that?!?!?!