Re: Rapid weight loss?
5 kg per month is one possible rule of thumb. But you can also use the 2% rule: Don't lose more than 1 - 2% of your body weight in one week. So if you've lost 5kg in 3 weeks, it should be ok, as long as you weighed at least 84 kg to start with.
(I am assuming you needed to lose at least 5 kg.)
Keep exercising, to make sure that you lose fat and not muscle. Assuming your diet isn't too strict, the weight loss should slow down as you approach your ideal weight.