Scales fluctuate all the time. There's a great thread about it here:
http://www.3fatchicks.com/forum/showthread.php?t=115826
Try not to let it get you down. Easier said than done, I know, but there is no way you gained three pounds overnight, so your body is just adjusting and before long that scale will start moving again. I have been gaining and losing the same pound for three weeks now, but my clothes are getting looser so I know *something* is happening even if the scale isn't cooperating.
It took me a long time to get over my obsession with that number, but now that I have it's a lot easier not to get discouraged and want to binge when the scale won't move. Keep at it! And join WW anyway. It's just a number, and chances are their scale is going to say something different from you mom's anyway. No scales ever agree with each other, which is why the actual number isn't as big a deal as the way you're feeling and the inches you lose.
Having said all that, I have a Tanita digital scale and it's awesome. Pricey, but very reliable.