b. sparky...

there are lots of reasons why you could have gained, most of them having to do with water retention for one reason or another. 1 pound is nothing to stress over (it might have even just been a temporary high for the week). Goodness...just from morning to evening every day, I can easily go up 5 pounds but it's always (well...almost always) gone again the next morning.
Weight loss is rarely ever in a steady downward line; there are bound to be ups and downs as your body lets go of fat, then holds onto water, then lets go of the water and the process starts all over again.
There are all kinds of reasons why you could see an increase on the scale and not all of them mean you have done something wrong. I'll give you an example. In December, I had surgery. I didn't change anything about the way I was eating, but gained 9 pounds in the week afterwards. Why? Water retention and swelling from the surgery. I had done nothing wrong, but my body was reacting to the stress and trauma of the surgery.
Now, that's an extreme example, but stress comes in many shapes and sizes. Too much salt can make you retain water. Goodness...sometimes you body just decides to hang on to water for no particular reason that you can find.
If you know that you are following your plan as you should, don't stress about going up a bit; it's normal and I'd be willing to bet that next week that pound will be gone again and maybe another besides.