Game charges interest on a 0 balance?

I paid the balance on my Game account on 28 October and was charged interest on 10 November. I phoned the customer help line listed on my statement and the lady I spoke to had no idea what I was trying to query - she kept telling me that my balance will only reflect at the end of November. I even held on while she could talk to her supervisor and came back with the same answer..... I then sent an email to the customer service department, on the 17 November, with my statement attached and a brief explanation, to date I have still not heard from them