(WFNN Bank)I just found out that I had a old Structure card that is showing as being delinquent?

The last activity from my credit report shows 1999, but in 2004 they started listing it as past due. I have not received a bill from them since 1999 (when I thought I paid it off). I called them and was told that they had a different address than my current one where I have been for over 5 years. My current address is on my credit report and can be found when I google my name. I will pay it, but want to know why they never sent my a bill and why did they reopen it after 5 years. I is adverely afting my credit and want to know if it is legal for a company to do this