It's completely normal to notice some changes in your weight without it meaning you've actually gained or lost weight.
Fluctuations of 1-2kg are normal
Menstrual Cycle
Your menstrual cycle can cause your body to retain more water during certain times of the month, causing a slight weight gain.
Salt
Excessive salt consumption can lead to fluid retention and this can show up on the scale.
Examples of foods with very high sodium-salt levels are Asian salads, chips, certain cheeses, foods in brine, etc.
Carbs
A high carb meal can also tip the scale WITHOUT CARBS FAULT, but fluids. With every gram of carbohydrates, the body will retain an additional 2-4 grams of water
Dehydration
You guessed it right! Just as extra fluids can increase your weight, dehydration can trick you into thinking you've lost weight. There are also many "fad diets" that, with the great loss of fluids they cause, make you think you've lost too much weight.
Heavy workout
A heavy workout, or if you're new to exercise, can cause temporary weight gain of 1-2 kg but this is water retention. Exercise causes structural damage to myofibers so inflammation results. This inflammation and build-up of fluid may show up as temporary weight gain after a workout.
Also, regular exercise can make your body store more glycogen to fuel exercise. Glycogen has to bind with water and that water adds a small amount of weight.
Other factors that play a role sometimes either showing a small decrease or a small increase in weight mainly due to fluids are Medication, alcohol or even some disease.