Crep

I feel rather crappy today. There are several main factors here, chief among them: I slept quite poorly, I I have a sniffle, and I've been exercising a lot lately.

One of the things I've never quite figured out though is how crappy exercising is supposed to make me feel the next day. How am I to tell when I am in the good "no pain, no gain" zone versus the bad "no really, I need a break" zone?