You should send the collection agency a letter, certified mail return receipt, requesting an updated statement. Who is the collection agency? I may be able to give you more specific suggestions depending in the collection agency.
Information on Avvo should not be construed as legal advice, as each case is different. For information about your specific case, please contact a consumer law attorney, or contact me at www.agrusslawfirm.com
Send them a letter demanding an accounting, and always send your payments certified or via some other way to prove that they were received and signed for by the Creditor.