Maintaining...

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That seems like a starvation diet to me.  Even though you are only 100lbs, I think if you subtract your exercising calorie burn from your normal body calorie burn, you are not taking in enough calories.  I am guessing that your 2 lb. gain was water weight.  No, you won't have to take in that few to maintain.  Eat a good protein breakfast in the morning and eat healthy in general--surely, you can take in at least 1500 calories a day, moderately exercise, and maintain your weight.

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What little I know about it is this:  if you tell the body that you are only going to take in so many calories--which is below what the body is used to operating wth as far as your basal metabolism rate--then the body decides it needs to burn fewer calories to keep you going.  So some people gain weight because they tricked their body into burning fewer calories.  That is generally not healthy.  So my advice is to eat a really healthy, protein breakfast after you drink a full glass of water to help jumpstart your metabolic engine.  I really don't think that you are taking in enough calories, especially if you are exercising.  Make sure you are getting enough protein and healthy carbs.  Other people would have some ideas about this if they choose to ring in.

Why were you eating so little in the first place?

It sounds to me like your metabolism has shifted to accommodate a very low caloric intake.  Nevertheless, such a low calorie diet is likely to result in health problems above and beyond the possibility of gaining a couple of pounds. 

You don't mention your height or sex, but I assume that if you put your stats into the burn meter you will find that you should be burning far more than 1200-1300 kcal per day.  Even if your body is not presently burning as many calories as the calculator suggests, you should still increase your intake so that your metabolism can recover.  You may gain some weight at first (some of which will just be water), but once your weight stabilizes you can lose whatever you gained the healthy way with a modest caloric deficit and then eventually return to a reasonable maintenance intake.

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