Gosh that is not an easy answer without knowing anything about you. Gaining weight depends on so much. Before anyone could even begin to answer you ....Is it genetic? are you young/old, healthy..what does your doctor say, are you athletic, or sedentary? I mean the list is endless. My girlfriend five feet tall and a size zero until after first pregnancy then she said her Italian genetics kicked in and she ballooned up to 180lbs and stayed there. So you just never know what is going to happen or what you are programmed for. If you aren't ill maybe you just want to celebrate your thinness. But whatever good luck.
