Charge of interest months later

So i purchased a cell phone in December on 6 month installment plan. I now get my end of September bill and im still being charged interest (apparantly you get charged 6month with interest) on cell phone purchases. I know its a minimum amount but how many other customers are you guys *********** with these low amounts? I have paid you guys the phones amount about 6 times over in the past months of 2017. Never ever paid a day late or a R1 short on my account and this is what you get for being a loyal customer.