I get this way constantly - like umm, right now. ;) When I'm busy, that's when I have the most energy and motivation and I can barely sleep. When things are slow (like right now) I feel incredibly sluggish, both physically & mentally, and never stop feeling tired.
If you aren't up to being productive, then let it be. Read a book, take a walk, watch a movie... don't feel pressured to do ANYTHING creative if you're not up for it. Sometimes you have to take advantage of the Internet and let it sell your items for you, and wait for the mood to pass.