Weight loss is not only about creating caloric deficiency always, as the weight you lose every week may not be the same. 

You may reach a point when you will notice hardly any weight loss for week/weeks followed by sudden loss, which is more than you expectations (>1.5kg). This kind of loss is also known as whoosh effect.

The main reason behind this erratic and unpredictable loss is water retention. When you lose fat, body releases water and fat loss will be visible only when excess water is flushed out of the body.

Prolonged caloric deficiency may cause water retention. It triggers cortisol secretion (stress hormone), which has few unwanted effects in the body including water retention.

Lot of work out, lack of sleep, electrolyte imbalance, low protein diet can also be the reasons behind increased cortisol secretion. 

It's advisable to take a meal break in-between if you are on very low caloric diet. Try to eat potassium rich and low sodium meals. Get time to relax, sleep and drink adequate water.