I am being accused of using a credit card without permissions.

I had a man I have known for 10 years living in my house 7 months ago. He is mentally unstable and I had medical power of attorney on him. He got upset one day and pulled out a gun in front of my children and said he was going to **** us. I told him to leave and called the cops. He went to jail. While he was living here, he had paid my satalite bill, phone bill and bought himself some things online. At the time he paid those bills for me with his card, I gave him cash. now he is saying I did it without consent. What do I do? An investigator wants to speak with me about it, I didnt do it.