Huge Bill from April 2018. Bill jumeped by 2666.67% in the past two month

• City of Johannesburg has been billing me +/- R700 per month since I purchased my house in April 2016, I have always paid and my account is up to date. In April 2018 my statements jumped to R16 253.13. I then logged a query on 23 April 2018 and I was given a reference number ********** 182. In the meantime I still paid the same amount as in March, which was R608.93. Today I received another statement with the a bill of R15 362.96. This means I now owe R31 007.16 (including interest). I am worried that if this will continues & they will blacklist me for something that is not my fault.