Start by taking out your loan contract and reading it. Under the federal Truth in Lending Law your contract should specify the amount financed, the interest rate, the number of months, and the total amount of the interest over the life of the loan.
Then use the following amortization calculator to see what those figures look like over the life of the loan. You can print out the amortization schedule that was generated.
http://www.bretwhissel.net/amortization/amortize.html
Then take a copy of that and a copy of your contract to the lender and ask for an explanation and documentation of why they want more money.