How can you lose 12 lbs in one week and gain in back in 5 days?

Last Monday I weighed 213, then on Friday when I weighed I was down to 205. When I weighed again on Sunday I was down to 201. I was so happy because I had never lost that much weight in one week. However when I weighed again on Thursday I was back to 212. How could this be? I've been known to retain a few pounds of water but never 12 lbs worth. What should I do? I have been so depressed about it.

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