Maybe your metabolism slowed down if you were cutting back on food at any point, and then you started to eat more, it could slow down causing you to gain weight. Keep exercising, you will never gain weight from usuing the treadmill. Do you use it on incline? Maybe you are gaining some muscle from the exercise. You will start to lose weight, it just takes time.

You could also be toning and building muscle which is heavier than fat. Measure your waist, keep up the treadmill workouts and check your waist again after a couple of weeks.