What is considered a common law marraige in Wisconsin

I lived in Wisconsin for 3 months with a person and had my mail delivered there even after I moved back to Oklahoma. So for approx. 7 months my mail was delivered at his residency. He Registered me for a credit card with his last name. And now I am receiving letters saying we are common law married. I may be pregnant with another mans child and am being informed if I don't get this taken care the baby will have his last name. Eventhough we wre not legally married.

Thank you for any help you may be able to give me.