Your body is made up of 60% water. So if you eat foods with too much sodium one day, you can see a gain in water weight the next day (even if you were on plan). Many things effect the scale such as sore muscles from exercise, stress, medications, sickness, etc Google "why the scale lies" for insight in to water weight gain. Our weight can fluctuate up and down all day long. If you were on plan, then it's probably water weight from something.