Lot of good advice in this thread.
A lot of the processes of the body - repairing of the muscle, carrying away of the bypdroducts of working out, burning fat, etc. use water. So even if you do everything right and are dehydrated you will see decreased results.
These shakes are a good source of protein, contain 0 sugar and are pretty tasty and satisfying. Low calorie too...
http://register.eas.com/OA_HTML/ibeCCtpItmDspRte.jsp?item=3405§ion=10779
And don't be so sure that you didn't train hard enough. Are you sore today? Some times it takes two days before you feel the soreness. That's why you can't really train the same body part too many times in a weak like Carnutzz said. I haven't gotten a good answer about why sometimes (and it varies for me) you feel soreness one day after or two days after. I have a suspicion it has to do with your hydration level (yeah I'm a broken record).
Good luck!