In order to properly answer your question additional information is necessary. There are legitimate charges that might apply to your account. Likewise, the debt collector might be violating certain laws. An experienced attorney can quickly identify the relevant issues and diagnose your problem. You do not state whether the debt was reduced to judgment, or whether the terms of your repayment plan include interest, fees and costs. You should make an appointment with a local attorney who specializes in defense of debt collection lawsuits.
In the meantime, let me give you a general overview of debt collection issues that might apply to your situation.
Illinois allows a judgment holder to add interest on the debt. Also, a court order awarding a judgment typically includes a provision that adds court costs and possibly attorney fees to the amount owed. If your case involves a judgment, look at the court order to see what amounts are included in the judgment.
If your case does not involve a judgment, the debt collector might be misapplying the payments. Again, more information is necessary. For example, if your debt can be classified as a consumer debt and if the debt collector is not the original creditor, your case might fall under the federal Fair Debt Collection Practices Act (FDCPA). There are other laws, both federal and state, that might cover this matter.
In any event, you should ask the debt collector to provide you with a written statement of the account history - one that shows all charges placed against the account, and all payments and credits entered in your favor. Compare this account history with your records and review the information with a local attorney who specializes in defense of debt collection lawsuits or FDCPA cases.
Do some research here on the Avvo site and on Google for "debt collection" and "FDCPA", making sure to limit your search to Illinois attorneys. You can find additional information on www.DebtLawNetwork.com or www.CreditLawNetwork.com. If you are near Palos Hills, Illinois, you might want to call attorney Dave Philipps for a consultation.