Weight Insecurities

For as long as I can remember, I've pretty much been underweight, or borderline underweight.  This has changed only recently.  (Thank god for freshman 15!  Or in my case freshman 10)  For example, in my freshman year of high school I was 5'5" and weighed 95 lbs.  Sophomore year, I was 5'6" and weighed 105.  Junior year, I was 5'7" and weighed 110.  Same for senior year.  Finally the summer before college I reached my goal of weighing a stable 115.  My weight fluctuated like no other, like 10 lbs at a time, and it was hard to get a stable weight.  Now after college I am a healthy 125 lbs (and still 5'7").
What really annoys me is when people comment on my weight.  I'm a female.  Haven't people been told not to comment on a girl's body size or weight?  I mean, in high school it was bad enough, but COLLEGE?!  Sometimes, people will just come up to me and comment on how skinny I am.  Is this an insult or a conversation starter?  Once someone called me "skinny as hell" right in front of me.  What the hell, I can HEAR you.
Do people assume that I'm flattered by these comments?  I'm not anorexic, I'm not using these comments to fuel my skinniness.  I'm just...little.  And I'm trying to be healthy.  So what business is it to anyone?  Do people think they're making a difference to me by telling me this?  No, actually I'm just lumping them in with the other crude assholes that comment on my weight.  Just because someone calls my arms scrawny doesn't mean I'm going to change my picky tastes in food.  They're really not making a difference.  I've heard it before.

The advantages of being skinny: infertility (I hate kids)
The disadvantages of being skinny: America is an obesity-centered culture.  Everything is revolved around people being too fat, and not acknowledging that health needs differ among different people.

Even Spanish models are being fattened up.  When they passed the regulation that models have to have a BMI over 18, I was too skinny to model in Spain.
Why is it that when clothes advertisements claim to cater to "every body type" they only make clothes for overweight people?

Do people treat overweight people the same way they treat underweight people?  I have never once heard a conversation start with "Oh my god you're so FAT!"  but many people have tried to start a conversation with me by saying "Oh my god you're so SKINNY!"  What's the difference? You're pointing out a flaw in someone else's body.  Does it really make a difference which flaw you're pointing out?

And for the record I am not, will not, and have never been (willingly) anorexic.
I have, however, stopped eating due to depression, not because I see myself as fat.