If you look at other people's diaries and the weight loss challenges, you will see that a lot of us have the same problem on a regular basis. Some days we have big losses, others very small or none, and on occasion, we put weight back on. I know I do. Even though I know I have done everything right, exercised, didn't go over my calories, the scales still say I put weight on.
That doesn't always mean you actually did though. For example, if you had something with a lot of salt the day before, it could be water retention. For me, retaining water can make a difference of up to 5 lbs. It doesn't mean that you put weight on, just that your body is holding on to water. I don't know your gender, but if you're female, hormones can cause temporary weight gain (mostly again, due to water retention). Time of the month puts between 3 and 5 lbs on me every single time.
Usually, when I have a couple of slow weeks, some really good ones follow. So I know it's temporary, and that encourages me to keep going. Taking measurements is a good idea as well, since it gives you something else to see some success than just to concentrate on the numbers on the scale.