Question about flucating weight

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Hi-
How can my weight go up after a BM? I know TMI but this AM was my weigh in day and I get up empty my bladder and get on the scale. I finish getting ready (not dressed just hair and stuff) the urge hits for BM. I go then get excited cause I firgure I could be down even more get on the scale and I'm up 1.3lbs from my earlier weigh in. WTH?? Thanks and sorry for the TMI this early but there has to be a reason.