This is going as I expected.
The most important part is that this isn't just drama for drama's sake, it feels natural. Kyoko is experiencing a bit of jealousy and maybe insecurity for the first time. But Shintaro only has eyes for her, so once things are clarified it'll be fine. This is good character development for Kyoko if anything.
Also, very importantly, the entire friendgroup knows what's going on now.
Kyoko being insecure feels out of place right after we had that birthday chapter.
It feels pretty on point to me. It's pretty clear this is her first serious relationship, so she's never felt the feelings she's having right now. Plenty of young women in this situation would have these kind of reflections or thoughts seeing their first boyfriend in this light for the first time. It's especially clear because she's seeing Shintaro communicate with someone in an impassioned way. The author has been clear to show us that Shintaro and Kyoko respect each other's passions, but she doesn't really "get" his art in the same way as his fellow art students.