OCD and Irrational Paternity Fears

I posted a little while ago about my OCD and was so encouraged by the responses I thought I'd go into a little more detail to see if anyone else can relate. I have diagnosed OCD, currently pregnant, on meds and also in therapy. I went out drinking around the time I conceived and cannot get it out of my head that I could have cheated, not remembered and that this isn't my husband's baby. I was pretty shocked to learn that both my therapist & psychiatrist have both heard of this fear/theme, so just wondering if anyone else has experienced this? The medicine is helping to calm me down, and my rational side knows this didn't happen, but I still CANNOT shake the thought.