What was your first clue that your SO was cheating on you?

I have a major feeling that something is going on with my SO the last month or so. We don't live together. He is suddenly spending a lot more time with his "daughter" (in the past, he has told me that his son and daughter rarely want to spend the day with him); I used to spend 2 or 3 nights a week at his place, and now he very rarely invites me over. He never lets me know his plans for the day, like he used to. I just have a very bad feeling about this. He is so vague about things! We were also supposed to go on a trip together soon, and he very abruptly didn't want me to go with him.