Maybe your wife found some very suspicious evidence or you are acting suspiciously, even if you did nothing wrong. Sometimes she might be acting this way just to get your attention. Maybe on a normal day when everything's the same, you don't pay attention to her, and she had to resort to this sort of tactic to get you to notice her. Just my theory... :)
What makes her think you are having an affair? And since when she is susspicious? Woman's intuition is usually right :) Maybe you two have disconnected. Maybe you are thinking about that new colleague that came in your office a couple of weeks ago and though you haven't done anything your wife senses the presence of another woman.. Think about it!
There are a couple of reasons why your wife might be suspicious, even if you’re not having an affair. One reason might be (let’s hope not) that she’s having an affair herself, and is using her suspicions of you to deflect attention away from her behavior. The other possibility is that she suffers from low self-esteem, and worries deep down that she’s not good enough to keep you. Telling her that’s not the case won’t usually help with this, so save yourself the trouble there. Instead, encourage her to do things that will improve her confidence, such as making new friends or learning a new skill.