Over charge

I started the cycle in between the month which then my 30 days cycle would be refreshing on the 9th on 12th of each month. However, that was never the case. I used the service for 2 months which I was billed for and never did there after. Yet I checked this month and I have an outstanding balance of R1250. No one is willing to give me answers as to how that happened. I'm very disappointed.