Yuu intruded, she wasn’t invited, she shouldn’t have been there, she should be the one apologizing and yet. Even despite that, even knowing that, Umi can’t help but feel like she’s the one to blame for acting the way she did. She isn’t in the wrong, she has nothing to apologize for, she felt violated by someone she considered a close friend, all because of what? Curiosity?
It has to hurt a whole hell of a lot, especially when the guy you like can’t be all yours. She was only keeping up the foolish pretense of this being a secret relationship as though that really mattered, all because it meant she had him all to herself if even only for one day a week. Now that’s gone, even if she keep being with him alone every Friday, that won’t change the fact that the “secret” is out, and now the only person who she feels has the ability to take him from her knows.
Of course she would feel angry and down right unreasonable. She wasn’t mad at Yuu, not really, she was just mad at herself for being a coward unable to just admit her feelings and convey them properly. She said so herself after all. But just like Maki having the courage to overcome the one thing he was concerned about for her, now that she has no reason to keep it all a secret she is more than willing to express herself. Yuu was the only hang up she had, she was the one thing keeping her from revealing everything since even as it was, it felt to her as if she was slowly taking Maki from her.
Now that he showed the courage to act, she found it within her to act as well, it also probably helps that they’re alone once again.