Why do I gain all the weight back once I'm back in America?

I went on vacation to visit my relatives in Europe. I ended up losing 5 pounds in one month without changing my eating patterns. Now that it's been a week since I returned to America, I've already gained back 2 pounds. I haven't been eating any more or less than before. What's the reason for this?
Why do I gain all the weight back once I'm back in America?
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