Honestly, Tarumi just showing up out of nowhere and is now in love with Shimamura too feels so weird too me.
You two haven't seen each other in years... Also what did she expect? For Shimamura to just leave Adachi?
I like this manga, but it's honestly very frustrating at times
Hardly out of nowhere, this has been building throughout the series. She clearly loved Shimamura when they were little, now that she's old enough it's not remotely weird for those feelings to turn romantic. Meeting her again must've felt like kismet, and she's clearly been falling harder each time they've met, and there have been a number of such meetings at this point, plus phone calls and texts.
Hell, when I was her age, I had crushes on girls I basically never talked to. That's just part of being a kid. But the ones where you actually do know them well and love what you see from them? Man, that's where you feel like it's just true love, and at the time you think you'll never get over it.
She didn't expect Shimamura to leave Adachi. She doesn't know anything about their relationship, and I don't think she's got enough entitlement to expect anything. She was probably hoping Shimamura would be like "Oh, my girlfriend? I dunno, I'm not that into her." so that she could feel justified making a move of her own, but Shimamura's feelings are deeper than even Shima thought they were, so when Taru saw that, it just shattered her. Hell, she might even have been hoping to just give up gracefully, leave Shimamura with something to remember her by (the drawing) and step away, maybe even maintain their friendship. But it's not that easy once the person is in front of you.
I always felt bad for Tarumi, because I know how rough it is having a crush on somebody that's into somebody else, but man this chapter really drove home the pain of it.