Aha!!!

I figured it out, guys! Why I am so tired:
While Michael was talking about how hungry he was, it suddenly clicked... I don't eat enough. At least, I haven't since I got sick.
Being sick completely squelches my appetite, and my appetite is really hard to build up again afterwards.
Food....sigh. It's been something I've forced myself to care about lately because, frankly, I don't! For some reason, being sick does that to me!
And I already have a hard time remembering to eat even when I'm healthy.
Yep, there it is! I haven't had enough calories in my body to sustain it in even the most sedentary activities. That's bad. No wonder I didn't gain weight on my forced gym-vacation.