oddly high bill

My 2 year cellphone contract was due to expire in November, but cell c did not close it as per the end date so I had to call in to ask for it to be closed and that i be converted to prepaid. They agreed but said they will close it end of December and convert me to prepaid but they will still debit me on the 25 january 2016 for december usage. They debited me R889 for December now on the 25 january 2016, my account had never gone beyond R550 before. I dont understand where this R889 is coming from now, especially because i have been on prepaid since the 1st january 2016. My cell phone account is ***