'Vi' is doing funny things to Sumire's stomach. The same kind of funny things as Akira-senpai laughing at something she said that he thinks is funny, or Akechi-san letting the Detective Prince mask down.

...or, now that she thinks about it, Futaba-senpai saying 'Stop Talking Shit About Yourself Challenge!' when she knows exactly how difficult it is.

Oh.

"I. I think I would."