contract and cash

I was just the co signer, why is it on my report? 

Depending on the debt and the creditor, the open credit may show on both of your reports.  After all you are also responsible if the payments can't be made by the other party, so it can be shown as you using your available credit.  That means it is showing as part of your debt to income ratio, even if the other person is making the payments.  

The other reason it can show up is if the other person is in default of the payments (even if they didn't tell you or deny it) and now it's not just an open account, but an account in collection.   Accounts in collection can show negatively on your report for many years. 

A co-sign is a person who has agreed they are using their credit, to assure that yes a debt will be paid and paid timely and fully.