Dreams of conceiving while cheating

So, my fiance Billy and I are trying to have a baby. It's been three months. Last night I had a dream that he and I had sex, and I didn't conceive but later went to my ex best friends house and we had sex and bam, I conceived. The dream quickly turned into a nightmare and it felt so real that when I woke up I was confused why Billy wasnt mad at me, I almost confessed cheating on him because it was obvious he didny know...Later I realized that it was only a dream.. But I wonder why I dreamt it. Michael (my ex bestie) and I nearly hate each other now. I've never even thought of cheating on Billy...

Anyone think they can help me understand this dream?