Yeah, I'm there with you. I stupidly did a Bodyattack class yesterday - my back hurt the entire time even though it doesn't normally, thought I was going to pass out when getting down to do planks and was generally quite embarrassed at how crap I was. Today is even worse, didn't want to get out of bed and all I want to do is sleep.
I've discovered I like black coffee, albeit quite weak and with lots of cold water - if I'm not actually more alert at least I feel it lol.
8 days!