billed 500 though account not owing

I have an account with you an I paid it up, never been on arrears or overdues but I was shocked to see that I owe you over 500 after I paid up the account, how possible is that? Where does such a high interest come from, I don't accept this as it's not cpa compliant and if you want to retain customer this must be stopped as it's day light *******... I need my money back or this is going further than this and you will loose client Base