There are plenty of reasons for a fluctuation in weight: sodium intake, hormonal changes, over/increase in exercising, etc.
Basically, try not to get too hung up on daily scale changes. Focus on the overall trend.
Even though I weigh daily, I only officially count a weekly weigh-in, and I tend to look at my average loss for the month to see the trend downwards.
If you haven't done anything to actually cause the gain (like overeating in the past week) then just relax and keep doing what you're doing. Sooner or later the scale will catch up.