One slower week (especially one that has all but a 1/2 pound of weight loss) is probably a bit too early to start calling plateau.
In the early part of the weight loss journey it is, for many people, very easy to lose 2 or even more pounds each week, every week. This gives a false impression that as long as we "are good" and keep up the same level of activity and intake, that this will continue.
However, weight loss just isn't something that you can program into your body. There are plenty of weeks (probably about a 3rd at the moment) where I don't lose any weight at all, or even gain a pound or 2, then the next week I suddenly drop 3, or 4, or even more pounds. You have to factor in hormones, water retention, "waste" retention, muscle development, and probably 4 or 5 other body functions that I don't know about that have an indirect effect to your weight as well.
This is why I've switched from focusing on the scale and really am trying to "feel good" based on how close I've followed my ideal lifestyle. I know
that the eating and exercising I'm doing now will lead to loss over the long haul, (and more importantly, better health) no matter what happens this week... so the weeks when nothing happens scale-wise don't bother me. Only the days when I "don't feel" like following my plan and give into those feelings do.
I know I've posted this before, but I really do this, and I think it's part of the reason for my success so far: everytime I am about to get on the scale I always say to myself "I can only control my eating and my exercise".
Just keep at it no matter what games the scale plays and you'll see the benefits happen.