Account does not go off after payment.

Since January 2015 I owed R****,**. then I pay February 2015 and amount, I payed March and amount and I payed Today 02/04/2015 and amount. so the last two month I have been paying R400.00 more on the account so that I can finish my account on 1 May 2015. when I went there today to pay and they told me my amount due to still pay I was surprised. then I asked the lady how so because I have been paying R400.00 extra to finish my account on the 1 of May they say on every the 5th of every month they add an amount of money to the account. NOW is Pathetic and F*** up because I don't know of anything added to my account. so the amount I still need to pay is R1700 and according to me I still ow them R1000. so how pathetic can they be by *****ing my money? is that even legal?? Can I Take it further?? and so by they way they cant just add and add and add money and I just have to pay pay pay!!! USeless