It could be a whole lot of things from too much sodium to hormones. Plus, the fact that your already at a low weight and don't have much to lose can make it much more difficult. You may need to get strict with your calories and make sure you writing every bite, lick and taste down honestly. If you don't exercise and can, start. Exercise is your best friend when you don't have much to lose and will help you keep the weight off (not to mention toning and heart health). If your already exercising, switch things up. Up your intensity or duration. Try different workouts to use different muscles. You might also think about focusing on fat loss and not necessarily just the scale. You can actually stay the same number on the scale but go down in sizes with eating really lean and exercising (plenty of lean protein, lean dairy and fruits and veggies). WHAT you eat can be as important as HOW MUCH. Also google "why the scale lies" for insight in to water weight gain.