I feel like Shimamura just hit puberty and stopped being as social as she was before, rather than having some dramatic falling out with her friends. It just wouldn’t fit the mood of the story. Shima is definitely displaying some gayness towards Adachi though (pro tip: no straight girls thinks about playing with another girl’s lips), although she’s definitely so deep in denial and fear that it will take a while for her to come to terms with it (as some other comments confirm as well). I was pretty much the same way. Managed to convince myself for a while that I hated a girl, when I really just had a crush on her, and i was way too scared to admit when other girls liked me. I can really relate to Shima on pretty much all accounts, including her views on relationships and how they seem like burdens. To me, it seems she’s just scared of expectations and the burdens of being in any meaningful relationship, let alone one with a girl.