I imagine there a lot of "just dump her, and move on" responses, and I agree, but give your self a little time for introspection first. Did you do anything that might have pushed her away? Has she been subtly hinting (or outright telling you) that she was losing interest? Relationships are hard man, and keeping a woman interested is probably the hardest part. Most you will tell you, or at least you warn you first, that they are getting bored and if you don't do something different (if you want to keep her) then that's on you. Hell, it's not hard to notice even she's trying to hide it, at least if it's someone that you know pretty well. Bottom line is that it's unlikely this came from nowhere, and while that doesn't excuse her behavior (cheating is cheating, she should have just broken up with you), learn what you did (or didn't do) to lose her interest for future relationships.