How tall are you (barefoot) - about 170cm ?? (5' 7")
Skeletal structure ? (small, medium large) - perhaps medium???
Can you grab a HANDFUL of fat from belly area whilst standing up straight ?
Loosing 2 to 3 kg (about 5 lb) is normal in the first week or two normal on almost ANY diet (you're releasing retained water from the body)
After that Loosing 0.5 to 1 kg (about 1 to 2 lb) a week is normal is you have excess fat (stuff you can grab with you hand) - this can slow down or even stall after a while.
When you get to the point you have to 'pinch-and inch' if body fat with fingers and not 'grab-whole-handed', then mostly like your body would have already slowed down weight loss.
You can be normal weight and be over weight on fat AND underweight on muscle (at the same time)
To me, underweight is underweight on MUSCLE.
In a everyday practical sense I RARELY see anyone 'on the street' (or anywhere) underweight on body FAT - meaning I don't think "they could use more body fat to be healthier".
Underweight people I see have always been underweight on muscle.
'Athletic' people I see are always "underweight" on body-fat (by "normal" standards) and carry a good amount of muscle (sometimes plenty) so their weight is very good (perhaps even 'heavy') - again I don't think "they could use more body fat to be healthier".
Many overweight people are badly underweight on muscle (the high percentage of fat they carry hides this reality).
It is possible you're underweight on muscle if your too 'wiry' (if so eat more good stuff and lift heavy things)
But my GUESS is you're not underweight on body-fat (and probably not muscle either)
Someone (anyone) saying to stop loosing or you're too thin (and they're heavier than they 'should be'):
Could your loosing weight be a slight threat to them?
Do they have a distorted idea on what is healthy weight?
Is your good health/eating habits, highlighting their not-so-good habits?
Short answer is you're not loosing weight too quick, just ensure your always carrying enough MUSCLE weight.
Sadly there's no point in trying to 'save' EVERYONE around you (it will just end in frustration for you and annoy them) - though occasionally you'll have people come to you seeing that you've easily maintained good health/weight over the long term.