Is it normal to loose 10 lbs in a week if you try?

I'm 5'10 and last Monday I was 171.2 lbs. The heaviest of my life. So I was very good this week and didn't fuck up at all. I made black bean protein brownies as a dessert cause I was craving sweets. I went to the gym 3 times. I got 10,000 steps most days. And today I'm 161.2. Crazy I'm just wondering is that pure fluke or maybe lots of water weight? MyFitnessPal says a max of 2 lbs a week so I'm just curious. I know it also depends on your weight initially.

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