Girlfriend might leave me?

Me and my girlfriend are now 3 and a half months together and it is a long distance relationship. I always trust her and had full confidence that she would never leave me until yesterday. We were talking about past relationships and he told me " remember how I said I never loved another boy I've thought about it since I said it and I think I did my first" and then she told me that she does not regret her dating him so I asked why and she told me how he made her feel... He was a great kisser etc... Because of how he said it, I think that she actually loves him all over again. The thing is that their is a school dance and he is volunteering for part of it but later she told me that her first bf might be their to ask her to date him once again. At first I was confident that I had nothing to worry about but now I am really scared. All I want her to do is be happy and If I can't do that from the distance, then I don't care her going with him and leaving me. But what do you guys think, should I expect a breakup and if so, why is she not breaking up now?