Continuous nightmares of my fiance cheating on/leaving me...?

My fiance and I have been together for over two years, and engaged for about a month. About a week after we got engaged, I started having these seemingly realistic dreams of him leaving me. I would wake up crying and call him (because we live a few hours away because of his job and my college.) He always tells me that he will never leave me and that it will be okay. About a week ago, I began having multiple nightmares a night, and remembering all of them. The first one would be of him leaving me. Then, the next two would be of him cheating on me, with different women each time. He promised me, and always has, that he has never cheated on anyone before and that's what his family and friends tell me, too. What does this mean?????? I've always had the trust issues with any guys I have dated, but my fiance helped me get over that. Now, I feel like it is coming back.