Bill suddenly went up with R400 pm (No Upgrade done)

My Account are suddenly R400 more per month than usual, I don't receive all my Invoices like I used to so I'm not able to see where the difference come in - I phoned customer care a few months back to change my account so that I can receive all invoices every month...still do not, I only receive a statement with my main number's Invoice. I want to know how Vodacom managed to bill me almost R400 more than usual. My account is normally R4572.28 now suddenly its above R4800. I Need answers