Unless you have been eating a lot (and I mean A LOT), a kilogram of fat mass gain is generally not a realistic figure in a matter of days.
Scales can be your own worst enemy sometimes - not because of your weight loss, but because they can be extremely inaccurate. Apart from weighing the amount of fat you are carrying, they are also weighing your fluid retention, your last few meals, your muscle mass and everything else in your body.
Did you jump on these scales at the same time, under similar conditions? Your body mass will fluctuate throughout the day. Later in the day you will have a tendency to hold extra fluids and thus increase your reading on the scales.
Meanwhile, in order to achieve your weight loss goal you will need to integrate a healthy eating plan with an exercise plan. A 'diet' alone will not produce the true results that you are capable of. By combining healthy eating with exercise, the two elements will work together symbiotically.