Winter felt her emotions taking over her as soon as she saw Marcellus.
He was alone out here the whole time. It was not only the sight of him being on his own like this that stirred her feelings, but it was also how he looked more distant than ever - perhaps, even sad. Him not appearing in the event hall at any point was enough to tell her that he did not want to be anywhere near her. Knowing him, he would have already bragged to her by making a grand entrance on the stage and revealing himself to be the sponsor.
She already had a strong feeling that he prepared all of these as a birthday surprise for her, but she did not want to admit that because she felt like she was being arrogant or assuming. Then again, there could be other reasons why he sponsored a public high school's prom - it could not possibly be about her all the time. However, it seemed like it did turn out to be about her, after all.