If you go into the "weight loss support" thread, they are right there at the top. One is titled "Water Weight and Scale Fluctuations" and discusses it a bit. A quick Google search will show you a lot of examples of it as well.
Anyway, that would be my guess... and it's totally normal.
Assuming that your new exercise program isn't causing you to eat more calories and therefore negate your workout calorie burn, water weight is the most likely culprit, especially if you are lifting weights.