After Hikari witnessed Yami kissing Yuu she ran off and waited in a hallway near the gym.
She witnessed the slap too, and never thinks awarely in the slap, even if, to be fair, she thinks several times this is probably a misunderstanding, but like Eriri before her, Hikari chooses being egoist instead ask Yuu for explanations.
Yuu assumed Hikari was upset because he had promised to hang out with her after her shift so he apologized and said his phone died. That's obviously a lie since Hikari's text show they were read earlier by Yuu.
Yes, this is the reason why I praise he is apologizing in his LINE messages. He definitely screwed up here and damaged Hikari trust on him.
This causes Hikari to start doubting Yuu in addition to what she saw in the classroom.
Later, during Yuu's confession he mentions he's always loved Hikari - based on what Hikari saw earlier, she took this as, maybe not an outright lie but a half-truth. It's this stuff that she's calling out when she calls him a liar.
To be fair, is not easy explain how you entered in a relationship with other girl who literally swore make you forget your previous crush. This is the reason why is so important when Yami criticizes Hikari because without her relationship with Yuu, Hikari never would have noticed him as a boy, and Hikari didn´t deny it.
And of course, this is why Seki calls all this "accidental two-timing". Accidental polygamy where literally Yuu´s (still) theoretical girlfriend gave permission to her best friend to date him.
This is the point where Hikari had to realize -and only unawarely she did- Yami already knew about her... and already HATED her before their first meeting. Even Haru and Yuki realized how Yami attitude changed when she discovered Hikari was the Yuu childhood friend she hates so much.
She's standing there in front of him thinking about confessing and then she flashes back to Yuu and Yami kissing and decides not to. It's not that she was a coward about it, she had legit reasons to abort the confession plan.
She had legit reasons, true. That doesn´t change she chose the worst moment ever to abort the confession plan -and later to literally insult and PUNCH Yuu instead ask calmly for an explanation in a moment where he seemed disposed to talk and even gave some hints about Yami if you read carefully-, and she, in retrospective, should have accept the idea of Yuu of returning to home instead force him to going to dance.
Equal as him, when Hikari tries to be proactive, she screws up. So, she is a coward who become afraid in the truth moment. In comparison, Yuu at least acts with more common sense in this kind of unexpected but decisive moments.
In the novelization it goes further with her wondering what was going on with her best friend and her crush. It makes sense that she'd want to talk to Yami first before doing anything but she's interrupted by Yuu confessing to her.
In that case, again, she had to say it clearly to Yuu instead carrying him to the gym and create the situation for a confession. Even more when Hikari bloasts about she being open and disposen to listen. Also, re-reading the novelization, she didn´t seem really looked Yami. She just encountered Yami in her classroom for casualty.