Well I weigh approximately 182 lbs. Yesterday I weighed myself before I went to play tennis and I was 182. I came back from tennis 2hrs later, and I was 178. Then today I weighed myself before I went to the gym, I was 180. When I got back from the gym, about 90min later I was 182. I didn't eat anything, and only drank water. Why does my weight fluxuate so much?