Discover Credit Card Complaint

Discover Credit card department,

Billing statement South Carolina

I called my credit card company and asked why I have paid over - a month and only - go toward my balance. The women responded that I 'm being charged interest even though the account is closed. I asked her how can they do that she said she do n't know. I just wanted to know if they can do this?? I have been paying back the money i owe for over a year now and my balance was - when I started paying back.. it is now -. Please help me because if they are not suppose to be adding interest I would like to know. I have every intentions of paying back the money I owe but do n't want to be cheated while doing it.