RBS Card Services Stolen chump change Internet

I had a credit card balance of $8783.99 with RBS with an 8% interest rate. I was sending $1000.00 per month payments and on time. Due to "economic conditions" per RBS they suddenly raised the rate to 18%. I borrowed from a family member and paid them off on 11/27/09. I received a confirmation letter that the account was closed with a zero balance on 12/01/09. On Christmas Eve I received a bill from them for $52.57 and it was interest from 11/17/09 per supervisor Todd . I asked why I was told the balance was zero when I called to request confirmation letter. He didn't know but I had to pay the $52.57 was his reply. I want to know if this is legal and would suggest to anyone reading this to never do business with this company in the future. Thank you.