Losing is hard

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Hi I am Sue
I am 50 years old and this weight gain is driving me nuts. I have gained 30 lbs in 3 years and I hate it.
I joined Goodlife last October and do at least 50 minutes of cardio plus the fit circuit. I started using this program 3 weeks ago. I lost almost 6 lbs and in the past week I saw my weight go back up almost 3 pds...why???