RCS overcharging

To RCS, how is it possible of I paid my account more then the settlement amount, that I'm still charged interest? When I paid my first of three settlement amounts RCS contacted me and said they are increasing my limit, how the hell is it possible that I owe against the increased limit amount. Original debt R 30 900 and paid to date R 32 200 and now receive a statement I need to pay R 131, if your app (Coricraft) indicated I over paid R 34