Raging hormones, water retention, irregularity, salty and heavy meals can all play havoc with our weight, which is why it can vary up to 2 kilos in 2 days! Weighing yourself constantly while you're trying to lose weight is a fatal error, not just because your weight will fluctuate but because it can completely demoralise you if you're not losing weight! The scales only give you an evolution of your weight, not the true picture.
Weigh yourself by all means, but once a week only. Make sure you always do it at the same time each week, naked, for a more accurate and more useful analysis of what you've lost.
It might also be a good idea (and an excellent motivational tool) to take your waist, hip, bum and thigh measurements every week to see where you're losing the weight from!