How Should I Act Towards My Ex-girlfriend?

OK, so we broke up about a month ago and she lives down the hall from my dorm room, we have been talking a little bit. We never said anything more than a question. But recently I had been under a lot of financial stress and I needed someone to talk to, when we were going out she would always talk to me about these kind of things or whatever was bothering me and help me with it. She helped calm me down and we haven\'t talked since.

So today her best friend was hanging out with my roommate in my dorm and my ex walks in and asks where she is because she has been trying to get a hold of her. I didn\'t really know how to act or what to say, she caught me off guard. I was chatting with her a little bit but all of her responses were stuck up or she didn\'t care. I don\'t know if I should just remain calm and be friendly or should I be mad, I have no idea how to handle this. I have always tried to be a gentleman but towards the end of our relationship I got over how we would argue a lot. I do get mad when I think about our relationship sometimes and I really can\'t stop thinking about her. Its really bothering me and I just really need some guidance to getting over this.