wrong amount billed

i have a query regarding the amount of money ndue to VVM. in Jan the balance to the due to VVM was r853 and i paid 400 and in feb when i asked them how much was the balance i was told over r500. i don't understand how an amount can be almost twice in amonth when i have made payment. i feel we being taken for a ride when we try to be responsinble citizens and pay our debts its like we being taken for a ride. i need that clarified as i would like to close this account as it a pain in the neck