Late fee for not paying late fees even though it's not in the lease?

Before anyone starts judging, let me provide some context and evidence—this property manager is awful. She messed up the original balance I was supposed to pay when I moved in, which included a $1200 deposit and the discounted first month's rent. Instead, she tried to charge me an extra $1200 on top of the original $1900 balance, totaling $2400. I had to contact the landlord to get that corrected. I have text message proof of all of this.

I paid my April rent on March 31st, so everything was up to date as far as rent. With a crazy first electric bill ($215 deposit + electric charges) then other major bills I couldn't cover the late fees just yet on the account. The only charges on my account were two late fees from February and March (which I didn't dispute, even though I wasn't late on April rent). Then, she added another late fee, even though the lease clearly states that late fees only apply if the rent for that specific month is late. It seems like this property manager and landlord are just trying to make money off late fees. How can I move forward with this?