Thank you Fulgour, Zorya and Chronata for sharing your thoughts on this strange observation
Zorya - what makes you think, those are negative energies being washed off? We do carry positive and negative ones

could it be that negative are more loosely attached?
Update for the experiment:
yesterday: nothing happened. No weight evaporating before and after shower.
today: yesssss!!! it happened again - 2 pounds are gone after shower
Another observation - I walk 30 minutes outside before shower. Yesterday I overslept and missed the walk. Could it be while I walk, some energies loosen their grasp on me and let go during the shower?