I've seen a lot of people posting similar complaints on here, and I think it brings to light several major issues with our society and weight. First is the fact that a lot of people have brought up here, that "normal sized" or "plus sized" people will often tend to discourage others from losing weight. It's basic psychology of self-validation and lifestyle-validation. Other's weight loss can call our own life choices into question, and we sometimes make those comments from a selfish place.
But, secondly, we also have to remember that those comments don't always come from a selfish place. Opinions on what is "too thin" vary widely, and once you get under a certain weight, it is inevitable that certain people are going to think you're too thin (while others will think you're not thin enough). It's just the nature of opinions, and it doesn't mean that the people who disapprove of certain weights are doing so simply because of insecurities about their own weights. We all have a certain level of thinness which we judge to be "too thin," unattractive and unhealthy looking, but it's far from universal. Parents are usually notorious for telling their children off about their weight, and we have to remember that they come from a different culture than our own, one in which an attractive woman meant something very different than it does today.
Which brings me to another point- "thin" people often face as much prejudice as "fat" people, only it comes from different places. For some people, a normal sized or thin person talking about dieting is the same as a fat person binging right in front of them. Are they feeling threatened by the other person's thinness? Perhaps, for some of them. Others might be turned off by what appears to be the vanity of the situation. The vast majority of people are uncomfortable around displays of vanity, and the implicit judgmental nature and self-consciousness of the vain. Now, I'm not saying that losing weight means you're vain, not at all. And many people wouldn't use the word "vanity" to describe it at all. By vanity, I mean generally caring about how you look. It's amazing how hard we have to try in order to look like we're not trying in this country! But in general, Americans tend to be put-off when people seem like they are trying.
And to my last point, the fact that we can't really judge ourselves. First of all, you have no perspective to say whether you're really "fat" or not. How often do you actually see your body? five minutes a day? Two? Most likely, it's even less. If you were another person looking at yourself, would you label yourself the same way? Would you call that girl fat? No, of course not. The BMI can be a helpful tool in general terms, but in most cases, it's highly flawed. Just look at this website, which shows just how arbitrary the system seems. Some of the women on it who look perfectly healthy are labeled as obese, while some women who look sickly are labeled as normal.
http://www.flickr.com/photos/7736776...7602199008819/
As you continue to lose weight, I'm sure you'll experience more events like this one that make you see just how complex weight issues are in this society. And all that being said, I think I can speak for most of the people at 3fc in saying that you have a right to pick the weight at which you feel healthy and beautiful, in spite of all of society's hold-ups about thinness or fatness.