It's all the crap in you legs that's built up and not been flushed out.

Whatever damage you do today takes 72 hours to break down, repair and flush out the crud (technical term). You are doing this, in part, everytime you ride (the flushing). If you suddenly stop you have 2 or 3 days worth of free radicals floating around your system burning holes in cell walls and causing general mayhem.

Do your normal ride tomorrow not at hammer head pace but at a pace where you AREN'T producing any more lactic acid. That should help flush the system out and you should be fine in a day or two.

BTW, this happens to me after I come off a really long and hard training cycle. There will be a few days where I'm not tired but the motor just sputters. I do a long recovery ride and a day or two later I'm good to go again.

YMMV.