How do I know if my husband has cheated?

Ok, this may sound a bit strange, but here goes. About a month and a half ago, I was on the internet looking at some adult content, and I ran across a video clip. In this video clip, they only showed the man's legs and torso. But, to me, it looked like my husband....afterall, I do know what he looks like undressed. I mean, everything looked like him. It really bothered me. I literally got sick to my stomach, because it felt like I was watching my husband with another woman. I brought it up to him that night and he denied it. He said that he's too shy to do something like that. But, in my mind, I kept thinking, his face was never shown so you don't have to be an exhibitionist to do something like that. I keep picturing it in my mind, and I'm not sure how to discuss it with him. I feel like I'm losing my mind because I can't stop thinking about it. I trust my husband, but at the same time I kinda don't...if you know what I mean. I can't explain it. At the same time, I feel like he's keeping something from me. Could this be it? I just need some advice on what to do. I need to know the truth once and for all, but I don't know how to get the truth out of him and know that he's not lying.