I'm having the same problem. My last scale went kaput during easter when I was off plan, so I got a new scale and I'm 5lbs heavier on it, so I don't know if that means I gained 5lbs or if it's just the fact that it's a new scale measuring differently.
Also, when I weighed 330lbs I went to the doctor and her scale said 308lbs, and I hadn't weighed in a while so I was ecstatic, so I got home to make sure I weighed that much on my scale and I was still 330lbs. Just goes to show how big the difference between scales can be.
Try not to dwell on it too much. I know it's frustrating, but it's also all the more reason to keep going.
Have you been measuring yourself? That might be a more accurate way of keeping track for now.
Oh, and JulieJ08 - that's a great idea, I'd never thought of doing that. I'm gonna have to try it.