I believe it's because she doesn't want Chan-il to think she was cheating on him. No one wants to be perceived as a bad person by someone they care about.
Ehh.. Cirrus basically used her while keeping her in the dark about his own feelings, Skylar's and Chan-il. He's very dear to her so it's understandable she feels disappointed on him.
Yeah, but this a perspective only WE readers know. Just by what he told her didn't justify such reaction. And disappointment is very different than that aggressiveness.
Just exactly my thoughts
I don't get why the girl is mad at Cirrus. I get why Skylar is. He didn't make her start dating Chan-Il. He didn't lie to them. He just introduced them, hoping they vibed, especially cause Chan-Il already liked her. Chan-Il shouldn't have even come to Cirrus about Rin, that's something you talk to your girlfriend about so Cirrus was right to say "go talk to your girl if you have questions" and gave her a heads up that situation happened. He could have stayed quite about it. If he had told her "yeah, I tried to fuck up your bf today but he was late to school" then I'd totally understand the slap. Currently, I don't get it.