Ladies love to talk about their feelings, and even if things are going swell in your lives, there's no doubt that if anything's bothering her, you'll hear about it. Which is fine - communication is the key to every successful relationship, and a chance to air out your feelings and find a way forward. But communication can also be a double-edged sword.
Every time she brings up relationship problems, consider how they're being brought up. Sometimes, it's true, she really is unhappy and is trying to give both of you a chance to mend things. But if the problems are coming thick and fast, and really very minor, then she could just be laying down the groundwork for a breakup. She could be justifying to herself the reasons why she's cheating on you.