Start by getting all 3 full credit reports from the 3 bureaus at www.annualcreditreport.com. In the detailed view for each lender, you will see a detailed payment history going back up to 10 years. (At least one of the companies codes their history green, yellow and red for easier identification.) If you find late payment marks on any of your reports, and you did not make late payments, start by contacting the lender. There is also an appeal process directly with the credit bureaus, but they will report whatever the lender certifies as accurate, so try starting with the lender(s) who are reporting the late payments.