My heart tells me to belive in both of them?

My really good friend passed away a a few months ago. Not only was she my friend but my fiances as well. After she passed away I come to find out that once she went to my fiances house to ask him for some advice. I didn't know about it because they both agreed not to tell me because they knew I did not approve of them to hang out together when I was not there. His mother assured me she did not go in the house and that she left quick. ( My fiance lives with his mother and she is the one that told me). I don't know what to think my fiance swears and assures nothing happened. My heart and gut tells me nothing happened either because I know the type of person she was. What should I do? Should I belive my gut?