I've been running 5 miles a day for the past week and I have eaten healthy..why have I gained weight?!

I wanted to cry when the scale said I gained some pounds. My mom said it's just my muscles adapting to the exercise and swelling with water. Is this true? and I know muscle weighs more than fat but it still scares me...Will it go away eventually and even out? I know I shouldn't be overreacting about this. I can feel my muscles being swollen too :s
I've been running 5 miles a day for the past week and I have eaten healthy..why have I gained weight?!
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