How should I feel?

I broke up with my ex a couple of months back, one reason being the fact that she cheated on me. Trying to be nice, and hoping for more of a chance with her (we hadn’t been going out that long) I said I forgave her. I soon got over her, as she went out with this other guy. Months later, she broke up with him. I had retained my friendship with her through this time. A few weeks over that, we were sleeping over a friends place, and we ended up sleeping in the same bed. She agreed that it meant something, and after that I started to like her again, as we talked for ages. I really got the impression that she liked me back, and I was going to discuss it with her soon. The problem is that very recently I found out that she had been sleeping with her ex even after they broke up, and to make matters worse, her ex says that they have gotten back together, which she never told me. I assume that they were going out when me and her slept together. I dont know how I should feel. I feel betrayed again, and used in a way, but did I make a mistake here to? and how should I react to her about it?