They met when he was feeling down about himself after a rejection, and one of the first things he did was put on a fake smile while thinking about how worthless and unlovable he was as a person, a facade that she immediately smashed through, which set the tone of their relationship as being honest, open, and uplifting. Last chapter, he realised that he had feelings for her, and immediately dropped into a spiral of self-loathing and doubt, and tried to once again face her with a fake smile while thinking about how worthless and unlovable he was. She tried to challenge him on it, like before, but he brushed her off this time, too scared to reveal his feelings, and in so doing, turned away from everything that was good about their relationship. She found this upsetting.
They both spent this chapter in a listless depression, where he's presumably decided that his fears have come true and she's realised how worthless and unlovable he is, while she (I'm guessing) is thinking something like "He's hurting but can't trust me enough to talk about it, I thought we meant more than this" or "I wanted to make him feel better about himself, if he's just going to feel this way because of me, then he's better off without me" or "I can't believe he'd shut off his own feelings like that without giving me a chance to respond to them, what am I supposed to do with myself now", or some other reason she's come up with to get upset about him not telling her why he's upset. We'll probably find out next chapter. I don't think it's especially complex or confusing, it's just super melodramatic in that particularly teenage kind of way where everything seems like the biggest deal in the world, even when it really isn't.