How did She Know?

I haven't watched the movie in a while, but a couple of possibilities popped into my head. The first might be that she was so worried over Kate, she realized how silly she had been and that she was not actually in love with Joe, but more infatuated. Perhaps she was also trying to live out some fantasy she had because of how her mother was (constantly with a new guy). I think maybe after she actually had sex with Joe, she realized that it was not love, but moreso a school girl crush or fantasy. If I remember correctly, the two really didn't know that much about each other, and even when they were together they didn't talk that much or seem to have a strong connection. I think a lot of us can relate to having that one crush on someone we find really attractive at a young age because they encapsulate some idea for us (the hot older guy in Joe's case, the hot unattainable, popular person in other cases, etc). If we actually got with these people though would it be love, or would we soon find we have little in common? Like Charlotte we would probably be happy very fleetingly. In Charlotte's case she bounces back to reality quickly with Kate's accident and probably realized her and Joe had little in common and she was really only attracted to him in a superficial sense. That's my take at least. I feel like it can be analyzed and viewed in a lot of different ways, but that's what I took from the film.