My mind is playing tricks! How to stop it.

So at the very beginning of our relationship I cheated in 2013.. I haven't slept with or seen the person since and I cut all contact. I'm happy in my relationship and I'm focusing on it. Fast forward a year and I fell pregnant in September 2014. Now I know the baby is my partners as I haven't been with anyone else and it would be impossible to be other persons as that was more than a year before.. But yet I still keep thinking what if the baby is not my partners.. What is this? I know I'm being stupid.. Is it guilt playing tricks?