Landlord Charged Me And Backdated It - Is This Illegal

Hello, I've been living in the same apartment in Wisconsin for about 2 years now, and this is not the first time this has happened. My landlord added a water charge to my account on January 11th and back dated it for December 31st and charged me late fees. I have screen shots from January 1st when I paid my rent that prove that there was not a water charge on my account and I had a $0 balance. They haven't shared any documents with me or given me a copy of the water bill or any notifications that my account had a charge added to it. Is this illegal? I've confronted them in the past when they have done this and they deny back dating but I have absolutely proof with the screenshots of my account ledger. Any advice on what I should do ? Thank you!