weight gain = change in body composition?

Since christmas I have been consistently not eating enough. Most days in january I would eat next to nothing and I lost quite a bit of weight (I am currently around 103-4 lbs). I have become slimmer, my thighs and tummy are smaller but i wouldnt say i look unhealthy. Thats the thing, I want my body to stay this shape but i want to start eating more because even now i am not eating enough (900 cals a day, 1200 at a push). Although i eat little i do eat healthy (oatmeal breakfast, fruit and veg, lean meats ect.) But i know i should be eating atleast 1300 (my bmr). I know after restricting calories like i have been doing I am bound to gain weight but (although that worries me slightly) i am more worried about getting a fat tummy and larger thighs again. If i increase my cals and gain weight will my body change drastically? I do dancing once a week for an hour and swim 1-2 times a week for 30 minutes. I also hope to start doinh weight training classes once i join the gym. So will the weight increase make me gain back all i lost causing me to go back to the shape i was (i was slim but my thighs were bigger and my tummy stuck out more) or will the weight go towards muscle ect? Im sorry if this is a dumb question but im really worried. Thank you in advance!