What does this mean?

I had a dream last night that I caught my girlfriend in bed with her ex-husband and I caught them after bthey had done it. In my dream, it had been going on for a long time. I remeber feeling so sad and lost after that throughout my dream and it lasted through the entire night. Sex is not so present in our realationship and that is my fault... but now I am starting to feel this is real and I cannot stop thinking about it... what does this mean???