Complaint on account being extra every month

I get a different amount every month when I have to pay for my account. I called last month and talked to a very rude agent asking why the amount is different on the app and invoice and she was rude about. I make sure I make payment every month I’ve never missed one payment but now I’m getting an August invoice and it says I owe them R1300 which I don’t understand how because I’ve never skipped one payment since I took the account and it’s always on time.