Boundaries

My fianace and I have been dating for about 3 years now and started a successful busines that involves meeting people and also giving out our numbers for contact. Recently, I found out that he has befriended a woman and has shared alot about himself. He admitted to me that it is nothing and that she is just a friend. However, last year we broke up for about a month and he pursued her. We are trying to work through some issues and are back into a commited serious relationship, but I found out that they still communicate and he still says it's nothing between them other than a friendship. Am I making this into too much, my instincts tell me NO!!, He' saying that im being insure and he would never cheat.