I forgot... you are pretty small to begin with, hence the low caloric intake. It's sort of low still, even given your size, but if it's working for you right now, so be it.
Eventually you might have to work in some sort of cyclical
diet where you alternate high/medium/low calorie days or start taking more
dieting breaks. Our bodies don't want us to get uber-light or lean.
As for your other point, adding muscle while you're
dieting and a female at your weight probably isn't going to happen. And if it did, it wouldn't be to the extent where you had a huge increase in metabolic rate.
