Why do I keep gaining weight, help me!?

I started exercising last summer to tone up. I was 117 pounds, im 5'7 btw. Now I am 127 pounds, and I am not toned! I don't know how I gained so much weight. I exercise 6 days a week, doing running, kickboxing cardio, body sculpting, and cycling. Also my diet, I eat really healthy most of the time. A lot healthier than my mother and sister who are the same weight and don't exercise. But I gain weight, and they don't. And don't say that it is muscle weight because my stomach it still fatty. The only way I seem to lose weight is if i go to 900 calories, and I know your not suppose to do that, if I go back to 1200 and try to be "healthy" I gain like crazy. Anyone who knows anything about nutrition, help!