increasing calorie intake & metabolism

Question
some background info: i'm 17, 1.56m tall and currently 45kg. that puts my BMI at 18.4. i've been restricting and eating an average of 900 kcals everyday for the past year or so and have lost 11kg in 12 months as a result (with some episodes of ups and downs in between).  my everyday diet is something like this: breakfast- 1/2 cup of rolled oats cooked with 2 tbsp of full-cream milk powder.  midmorning snack- 1 red apple, 1 green apple.  lunch- lots of leafy veggies, 1/3 serving of steamed fish, 1/2 cup of cooked spaghetti, 2 oranges.  Afternoon snack- 1 red apple, 1 green apple.  Dinner- lots of leafy veggies, 1/2 serving of pork, 1 serving of steamed fish and 1 potato. everything is cooked without the addition of oil. Most of my calories come from fruits as you can see. i also brisk walk/jog for 50 minutes everyday as exercise.

I think my body has kinda adjusted to surviving with only 900 kcals, with my metabolic rate slowing down, cause i have amenorrhea and i don't feel hungry if i eat 900 calories but feel bloated when eating more than that.  

now that i really wanna eat 'normally' again, but still wanna maintain my current weight, should i increase my calorie intake slowly to reach the ideal intake of 1800 kcals for my age (e.g. eating 1100 kcals per day for 2 weeks, then 1300 kcals per day for the 2 weeks and so on) or should i just increase to 1800 kcals straight away?  

p.s. i've read from various sources that a sudden increase in calorie intake will create this thing called hypermetabolism, which actually prevents weight gain despite the increased calorie intake. is this true?

thanks

Answer
If you are aware of your BMI, you know that you are at the very edge of what is considered healthy for your height.

I think that you are going to need medical help with this.

Restricting calories in this way can have far longer lasting and catastrophic effects on you body- much worse than a slowed down metabolism.

Think about what has happened already, your body has shut down it's reproductive system- that means your hormones and several internal organs have been changed.

The "hypermetabolism" you are hoping for is actually a neurological symptom,a glitch in the brain usually found in patients with anorexia due to terminal Cancer or AIDS. It speeds up their death.

There are ways to get out of this in a healthy and responsible way, but you cannot do it alone. Go see your doctor, or a psychiatrist. Then you can work with a nutritionist and a trainer to get a strong healthy body that you can be proud of. Trying to fix this on your own won't work

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