Used to Undereat...now worried about gaining!

When you starve yourself for a period of time, and then all of a sudden feed it more, your body will sorta Freak out and store that little bit of food for about a week or two, then when it starts realizing that the 1200 calories is normal, then it will start losing again.

A 1-3 pound gain is very normal for these situations. There are consequences for every action we take, this is simply that consequence.

Make sure you are NETTING 1200 calories, meaning that you eat your exercise calories back too, or you will still be eating at a deficit.

Put your scale away! I know it's hard because I weigh myself everyday to see what damage or success the day before had. But if you are serious about wanting to be healthy you can't base your fitness on the scale. Eat your calories like you should and then introduce the scale back into your life when you feel that your eating habits are healthy. Then you can use the scale for one of the methods to base your goal weight on.

You seem to get the gist of it, but just wanted to add a couple of things for you and others. Chronic lack of appetite can be a signal of severely decreased metabolism. A couple of threads that may help:

http://www.myfitnesspal.com/topics/show/175241-a-personal-view-on-exercise-cals-and-underfeeding

http://www.myfitnesspal.com/topics/show/231636-the-eating-when-you-re-not-hungry-dilemma

http://www.myfitnesspal.com/topics/show/186814-some-mfp-basics

http://www.myfitnesspal.com/topics/show/230930-starvation-mode-how-it-works

It is normal and expected to see a temporary gain when you increase cals after underfeeding. Usually within a few weeks that will stop and you'll start to see a loss again.

As for how to increase cals in a healthy way:

Planning is key. Focus on balanced macros (protein/fat/carbs) in each meal/snack. Start with making sure to eat at least 3 meals a day. Breakfast is important when you're trying to overcome a habit of irregular eating or underfeeding. Once you've adjusted to that, add in snacks. Then, gradually increase the amount of cals in each meal/snack. Just 50 cals more for each meal/snack adds up. If you know you'll be working out, increase cals throughout the day.

If you're not feeling hungry, a good way to increase cals without much volume is healthy fats from natural oils (olive/canola), nuts and nut butters (almonds, pistachios, walnuts), and avocados. Also, I would stay away from "low cal" or "low fat" options. Partly because you want to increase cals, and partly because those foods are usually processed and not nutrient dense, and the cals or fat they take out is replaced by sugar (real or substitute) and sodium - not a good trade off. A fruit smoothie, protein shake or chocolate milk are good cal boosters, because drinking cals can be easier than eating when you aren't feeling hungry.

Typically, it may be hard for a few days or a week, but your body will adjust to the increased intake and you'll find it isn't an issue for long. Eating more will truly help your metabolism, and your appetite will increase with it.