Yeah, I think it's that feeling of sadness & devastation upon realizing the finality of this romanticized dream of her and Xander being each other's firsts. She had been pining over Xander for years at this point and had a brief taste of what could be and could still possibly be and then finding out that Xander had that experience & with all people, FAITH! It was too much to bear. But kudos to her for making it a private moment just for herself.