I know it doesn't seem like much but it is still a loss Much better than STS or putting on.
Did you speak to your consultant? Is there any chance you think you were 100% but actually missed some of your HE's or didn't use enough syns?
You r weight will naturally fluctuate anyway, so one week of losing half a pound is nothing to worry about, it is the bigger picture that matters - your weight chart is still dropping, and that is the important thing
Best of luck, hope you can find it in yourself to be pleased for doing well, following the plan and losing some weight