Weight maintenance at anorexic weight?

I'm 19 years old, female, 5'10", and I weigh 110 lbs. I started out at nearly 170 lbs. in February of 2011, but starved myself to get down to 125 by November of that same year. That same month, I started seeing a nutritionist and following a 1200-calorie-minimum meal plan that she gave me, and I stopped exercising. However, despite all that, I kept dropping weight; as of yesterday, I'm down to 110 lbs. I live in New York City, and I do a LOT of walking and dancing on weekends, but it's still surprising: for the past month or so I've been increasing what I've been eating a lot-- some days I'll eat up to 3000 calories, and I'll have spans of a few days where my intake hovers around 2200 calories, but on average I eat 17-1800 calories (of course, I do walk a lot, so I burn a lot). I am lactose intolerant and a vegetarian, as well as somewhat of a health nut, and I eat a lot of lean protein (soy, protein bars, etc.), healthy fats (nuts), fruits, vegetables, and whole grains. It's not like I get these calories from junk, so maybe that's why I haven't gained. My question is, is it possible that my metabolism has gotten so fast that I can maintain this low weight on 2000 calories a day without exercising? Like, is it plausible to believe that I can eat this much and not work out and not gain weight? Does the fact that my calories come from healthy fuel rather than junk food make a difference? I think I like the way I look (even though I'm told that I look terrible and far too thin), and I don't want to gain, so I'm wondering if this pattern will make me gain and I should cut back a little bit for a while.

You need to gain at least twenty pounds. Period. This is a recovery forum, so you are not going to get many supportive replies when you ask if you can maintain an unhealthy weight. I don't know why on earth your nutritionist would have given you a 1200 calorie meal plan to follow. You should be eating at least 3000 calories per day and refraining from exercise until you gain weight. You are currently at a dangerously low weight and could begin to have related health issues at any moment. I know it's scary and you don't want to gain weight, but you really need to. Are you in therapy?

I am. Also yeah she was a pretty bad nutritionist.

A lot of soy protein: tofu, edamame, stuff like that. Also fruits, vegetables, whole grains, olive oil, and nuts.