Liked chapter 18 a lot, it's good to see the other point of view of this mess.
What I think is that Jin-ah, from her flashback and some moments in previous chapters, thought Heo Jina acted like that to all the other girls. In her flashback she was really bothered when Jina was playing with the other girls.
My guess is, after the kiss, Heo Jina was trying to make it seem like she wasn't romantically interested in Jin-ah to not ruin their friendship, Jin-ah got that as the literal idea (cause she's an air head) and thought Jina didn't think she was special and treated all the other girls the same, so she got a guy to be her bf and make Heo Jina jealous, but instead of trying to talk about her feelings, Jina decided to ruin Jin-ah's relationship and kissed the boy. Then Jin-ah was certain that Heo Jina did that (the skinship, the kissing, the playfullness) with anyone. That's why in early chapters she said she resented Heo Jina more than her boyfriend, and when they where fighting she said Jina was only having fun at her cost, making her feel special when she's not. Then Jin-ah went through the biggest internalisation i've ever seen, my girl suppressed her romantic feelings for Heo Jina really hard.
Basically, none of then talked about their feelings and this lead us to where the story began. It's interesting to read the story since the beginning with this new perspective.
Started reading the series thinking it would would a lame smut yuri like a lot of others, but what a nice surprise, this one is funny and has interesting characters.