Water weight gain from increased muscle repair. 1 lb weight loss requires you to cut 3500 cal. If you have a calorie deficit, it is not possible to gain fat. Exercise will not really significantly enhance your weight loss based on scale weight, but over time you will gain muscle, get toned, and change your hormonal balance positively. I actually first gained 10 lbs before I started loosing. Also, there will always be times when you do not loose. You need to get away of this instant gratification feeling and start to look at weight loss as something long-term. As long as you create a significant calorie deficit, you are going to loose fat eventually. Just keep on doing what you are doing and try to ignore the scale. Take measurements. Often they are more telling (and motivating) than your scale.