You do not weight very much at all, thou for your height it may not be dangerously low? it must still be very low.
One thing i am certain of is that you are starving yourself 2 small meals a day with no carbs is just not enough.
Here is a litlte secret i have if you want to maintain your weight. Cut out all sweets and junk food from your diet, but dont be scared to eat a lot, dont necciserly eat when ever you feel like it, but if you feel very hungry then eat, just make sure the food is low fat. You want to know why you wont gain weight? because you will start either running or cycling (outside, not in the gym) you will do it intensly and you will do it long distance so that it takes you sevral hours and you come back complety shattered. You will not gain weight easily if you do excersize to the point where you are totaly physicaly fatigued, i have been doing this for sevral years. It takes a lot of detirmination and you may not mange to do it at first, but if you find a given route of a decent legnth e.g. start with a 30km cycle and each ride try and beat your previous time, race against yourself like this and then increase the distance every so often, the easiest way to cycle (or run) intesnly enough is to race against yourself in this way.