Girlfriend acting weird - help?

So me and this girl have been dating (unofficially) for about 6 months. We're seniors in high school so we just went through the whole prom thing - which caused problems. She had promised this guy who she used to go out with that she would take him to her prom a long time ago. So naturally she kept her promise and took him, even though me and her have had this ongoing thing. Since he lives in a different state, he flew into town and stayed at her house for about 5 days. During these 5 days I barely got to talk to her. The first couple days at night she would text me "I miss you/I love you" texts and we talked on the phone a LITTLE, and then the last couple days we didn't really talk at all. Yesterday, the day he left town, me and her hung out a little bit and went out to eat. She was acting really weird the whole time for example not saying I love you back, acting sassy, she didn't seem interested in kissing me like she usually does. Its possible she was just in a bad mood. Anyways later yesterday I texted her and asked her if everything was okay because she was acting like something was wrong, I may have acted a little crazy in asking her about whats wrong. Anyways she told me that "we're fine" and also said "we need to chat about earlier today". I just don't want it to end up being some huge argument about it, I honestly don't care if she hooked up with this guy, because technically we weren't dating or whatever, as long as I'm the guy she wants to be with. I really just want us to stop this weirdness phase and go back to the way things were about a week ago, because things were really perfect. Any advice as to how I can get us through this little bump in the road would be appreciated. Thanks.