Rapid weight increase?

Please can someone help me explain, that when i eat how big or small, I put on like 2lbs whenever I eat anything like a meal, whether healthy or not! I think this is the reason why I suffered or recovering from my Eating disorder. the fear that if i ate my recommended calories i wld gain weight so easily and become fat, so instead i chose to not eat! do i need to condition my body increase my metabolism, how can i do that, plz help!i have put on 6lbs in 2 weeks and i have been so good with my diet, i am 6stone, bmi 14, 5ft4....go to the gymn already 3 times a week, 30 minutes treadmill, 7 rower, 5minutes cross trainer and 10min weights and crunches! oh look after my daughter 10mth...feel so depressed at this rate i will increase my weight so quick how can i eat and not gain weight! i eat so less anyway plz help