You know I imagine if there was any truth to you having sex with another person, it would have happened before as well, in theory. A lot of people who really do this sort of thing do it more than once. You seem like a nice person who loves your husband and I am doubting very much the incident ever happened. Just a negative thought that has entered your mind. Your husband is the father of the baby and you have no reason to doubt this, other than a thought you have no proof to back up. So try switching thoughts. To the man you married and love as the father of your child. It will do your mind the world of good.