You need to be eating over 1200 calories a day in order to prevent your body thinking its not eating enough and sotring everything it can as fat.
Also the difficult thing to do is lose weight and build muscle at the same. Muscle not only weighs more than fat and needs more calories to maintain but also requires an excess of calories to build.
I am on a strick diet at the moment and have lost over 2 stone since the beginning of the year due to carefully monitoring what I eat and how I exercise. I have enlisted the help of the local gym and use the services of the dietician there so know that what I am doing is sustainable. I have now increased the calories intake in order to reduce the weight loss and increase muscle mass.
I would suggest seeking the advice of either your doctor, gym or dietician in irder to get the best advice.