Outstanding balance after 6 months!!!

My son was admitted to hospital on 30 July 2015 and the hospital said I should pay in advance. And so I did. The doctors were paid and so was the hospital. He was discharged 2 August 2015. I signed all the release forms and everything. Now all of a sudden on 22 January 2016 I got a statement saying that there is an outstanding balance of R1400! What the hell? Really? After 6 months from being discharged! I need answers! I paid!! I really do not need this!