The last time I lost weight, friends, co-workers and family told me the same thing. I believe it was because they were used to seeing me 70 pounds heavier and it was somewhat of a shock. Even when I was only half way, they started suspecting I was doing something drastic to lose the weight. Strangers never thought I looked "too thin" or "sickly" because they didn't know the former me.
Keep doing what you're doing. Lowcarbing will bring you to a healthy weight. People that are underweight often "gain" weight on lowcarb because it will take you to the healthiest weight for YOU.