A credit company can allocate an account as "charged off" if there has been no payment for a period of 120 days or more. But, there are some companies that initiate it after the 90 days mark. If you have had all the proof of your payments, you could have place the legal dispute and saved your CR from getting tarnished.
Shirley, check your bank statements and review all the dates of the payments made. If there is a slightest possibility of placing the dispute again, you can still do it. You have already made the payment that was required. Now, what you will be doing is that your file should reflect your actual credit worthiness. For this, any kind of accurate dispute if done will be helpful.
However, there are various classifications on reporting an account as 'charged off' to the CRA. Read the FTC website and learn more on this subject.