I don't quite get it either. I think it's that Momiji's all "pant pant pant it's sooooo hot" when it's just Nitori, and she suddenly perks up and tries to play it cool when Aya-chan shows up.
I think part of it is also the fact that Momiji didn't move an inch and just kept panting in the 3rd panel. She knew that Aya gave her the wrong issue so she just stayed cuz she knew she'd be back shortly. Nitori didn't catch on, so she was a little confused. At least, this is my theory on that situation.
Here, Bunbunmaru, latest issue.Are you alright?S-so hot~Eh? Eh?Ohh!?Momiji, after those are all distributed, let's go on a date♪!Any way you look at her, she's still a dog...Good afternoon! Today is hot as well, isn't it?Geez, don't say such weird things so suddenly!Hmmm-...Really, Aya-san is too careless.I'm sorry. I gave you the previous issue by mistakeI came to take it back.
