I was going to suggest jealousy too, but it depends. If you went from 11 and a half stone to 10 stone, then you're unlikely to look 'gaunt.' But if you went from 10 stone to 8 and a half stone, you may do. Depends on your height and build and all sorts of varying factors. My DH has dieted on and off over the years, (he is 5 ft 9 and a half,) and he has been between 10 stone and 16 stone - over the past 35 years. The best weight for him is 12 and a half to 13 and a half stone.
When he dropped to between 10 stone, and 10 and a half stone, he looked awful. He looked like he needed to be hospitalised! Gaunt, and haggard and very pale. Just doesn't suit him.
Same with me. I am 5 ft 4, and whilst I am supposed to be 8 st 5 to 8 st 10 according to 'experts' I look awful at that weight. Gaunt, and drawn, and ill. My ideal weight to look healthy and 'nice' is 10 st 2 to 10 st 11.
So yeah, it's possible you may look gaunt @Swedishh but you haven't said what your height is, (or what your weight was before,) so it's hard to know if a stone a half weight loss is a bit much for you.
Still, it was quite rude of them to comment anyway.