> Strangely, I usually put on more weight during Ramandan.

If you are starving, your metabolism slows down to conserve energy. I assume you eat heavy meals(high carb? sugar? oil?) when you break your fast(dinner? before morning prayers?). Slow metabolism, coupled with high calory intake when your body doesn't need it(if you go to bed after a heavy meal, what would your body use the calories for?) would be the reason for the weight gain.

Your body starts eating away at your muscles and storing as much fat as it can. That's why it's important to exercise.