Lily had been having fun. She loved balls and dances, and this was probably the last time she would get to have this much fun, at least for the next year or so. She had a wonderful time dancing with James and talking with some of her old classmates that had shown up. All in all, a fun evening, even if she was there to chaperone.
As many times as she'd missed it while in school, Lily felt eyes on her. It didn't take long to find Severus watching her. Why was he watching her? Hadn't he damaged her enough in school? She scowled and turned away, determined to ignore him. After a minute, she looked back, and he was still watching. Why? Frustrated, she walked over to a group of students to ask them about their costumes, some of which she didn't recognise. But he was still watching her. In all honesty, it had probably only been a couple minutes since she first caught him, but it felt like much longer. She didn't want him watching her.
Lily was irritated, mostly with herself for not being able to ignore him, but also with him. What right did he have? Granted, she was unusually unforgiving with him, but he was the one who had betrayed their friendship. He was the one who had separated them.
Severus may not have had the courage to approach her, but Lily, in her irritation and frustration, had lots of courage. She was not very far away when she made the decision to confront him, so it did not take her long to reach him.