Mood changes can point to more than just stressful working hours. Sometimes, people get restless or irritable because they are expected to be in two places at once. When the other woman demands more of your cheating spouse’s time, he may begin quarrelling with you more often so that he can storm out to meet her. If he’s not only picking fights, but is also bothered about stuff that hasn’t been a problem before (ie. your appearance, eating habits, intellect), it’s a clear sign that the problem lies not in you but is about another woman entirely. This is how cheating will start to cause a lot of inner friction and stress between the two parties.
In addition, in order to keep up the act of cheating, you have to concentrate on your dislikes about the other party. It is possible that by nitpicking and finding other reasons to ‘tear down’ the relationship, he will feel that his cheating is more justified and feel less guilty about it.