"Seeing her so incredibly delighted once again caused me to grow suspicious. No, stop. I shook those thoughts away."
She's totally doing her best to deny that Adachi might like her, and completely not prepared to deal with what that situation would entail. She's not been terribly overt about any sort of reciprocal stuff, but since Adachi is the one on her mind so much, and she has thoughts about playing with her lips and stuff, to me it's just her comin' round to the whole idea.
Yeah, Shimamura just seems like her denial is a lot stronger than Adachi. And shit, even Adachi is still kind of refusing to admit to herself that she's gay, even as she's asking a girl she likes out on a date. Shimamura's whole narrative is her immediately understanding how Adachi means the date request, getting nerves, and then desperately searching for some explanation for why it'd be no homo for her to accept a date with a girl. But also it never really occurs to her to turn Adachi down.
Like she's obviously nervous about their relationship getting more complicated, and then there's an undercurrent of gay self denial, where she's like struggling to contain her thoughts and contextualize things. She cuts off various too real lines of thought like 3 or 4 times, and she knows that the line about just wanting to go out with anybody on Christmas doesn't hold water, but she latches on to it as a reason why she can say yes without then having to like, redefine herself and her relationship with Adachi.
...Also, big yikes with the whole Mom walking in thing.