What do I do when I suspect that someone have used me as a cosigner without permission?

Today I received a letter from Sallie Mae saying that they had turned me down for a loan. I have a loan with Sallie Mae that has gone through already. Then in the very top corner of the letter it has a relatives name on it! I did not give her or her mother permission to use me as a cosigner! I have my own education to pay for so why the hell would I cosign for someone else?! What should I do? We have had disagreements in the past and enough is enough. How will this affect me and what action should I take?