Why would I gain? Help advice

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So I have been eating 1200 calories a day and exersising daily...my workouts and getting longer and more intense and I don't do the same thing everyday. I lost 5 pounds last week (my first week) and I got on the scale today and I gained 2 pounds! I just weighed my self yesterday and it was two pounds less. I am using the same scale and wearing the same amount of clothes. Why would I gain weight?

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