There was an edge to Su's tone when she said that, and edge that she hadn't meant, or even realized. It didn't help that she'd looked at Lisa very pointedly as she spoke. She knew Lisa was just trying to help, but Su didn't like being mopey or angsty. She'd tried it, believe her. It's what she'd been doing the past few days, or at least as close as she let herself get before getting frustrated. Maybe it just wasn't for everyone.
Now she thought about it, maybe the upcoming Quidditch game would be good for something more than just bragging rights. Su tended to get emotional negativity out through playing aggressively anyway - at the rate she was going, this game could very well be a bloodbath. As long as it was a bloodbath that ended with her hands around the Snitch and the second Ravenclaw victory of the season.
This joyful thought planted in her mind, Su's expression softened and she came a bit closer to Lisa before sitting down at her feet, cross-legged.
"Don't worry. If I need to 'express myself', I certainly will. I just feel like I've done enough of that for now," She smirked. "I'll save the rest of my storm and stress for the Slytherin Quidditch team in a few weeks."