The scale is
you know that, right?? Seriously, it is!
This happens a lot -- see the sticky at the top of the page!
One problem is that we use the scale to try to measure fat loss, but it's actually not a good tool for that. It measures all of us, and most of us is water. So our water fluctuates all the time and makes it hard for us to know what's going on with the fat (and the muscle...)
So, if your water fluctuates because of sodium, or TOM, or because your muscles are holding on to water after exercise, or a myriad other reasons, you may see the scale go up.
It's actually possible that you lost 1 pound of fat this week but gained 2 pounds of water. The scale looks to be up one pound, but you can't see the fat loss.
The only solution? Just keep going! Use the scale, but don't give it too much power over you!