You're body is literally eating a lot of muscle you're trying to build by going that low. Women who don't exercise should eat about 1300 -1500 calories a day.
At your size, age, and height roughly you should be eating 2100 calories a day without exercise. With exercise for weight loss you should be hitting 2450 calories a day. With at least .8-1g of protein per pound of body weight. Fill in with 20-35% of healthy fats. And then the rest from carbs (and healthy ones, maybe cut sugars, grains, starches, and focus on plants and some fruit). I eat 80% paleo so I've cut grains and processed foods almost completely and seen great results.
Do you sleep a lot? What kind of lifting schedule do you hit?