Have you asked her about it? Don't make assumptions. Be as clear as clear can be and the only way you can do that is by asking her. Look, everyone has the right to tell you yes or no but you always have the right to ask. Ask her if she's having an affair, ask her why she's so secretive and, yes, she may get upset but if she's not willing to have an open and honest conversation, that speaks volumes.
Remember: "When someone shows you who they are, believe them..."
Pre-Divorce Planning For Smart People