It's all down to water retention.
During your TOTM's your body holds on to a lot more water than at any other time during the month, thus "adding" to your weight (you can see from most people ticker's that they tend to lose less then their average during their "lady week" then more than their average the week after as the water comes back off)
Can't tell you why you retain water (well, I can, but I won't bore you with it

), but it is what happens.
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