How is this even possible

2 months ago after paying exact amount as detailed in invoice I was informed my account is R84 in arrears, this month after paying R50 more than stated on my invoice, I was informed that my account is again in arrears, upon phoning the agent I was shocked when he informed me my account was in arrears with R 1100 and that I must make a payment within the next 2 days. How is this even possible when my installment is R900 and I pay this every month on the 26th? What is going in Vodacom? Sort this out or I stop paying completely and a judge can decide in court as I have all my invoices and proof of payments showing I pay every invoice on time. You are violating my rights as a consumer