@sylvacoer It's probably a combination of surprise, shock, denial, trying to figure out what to do, depression and bewilderment. He discovers his girlfriend in an private, intimate conversation with someone she has led him to believe that she did not know, in the middle of the night, no less, and hears the guy, who also led him to believe that he did not know his girlfriend, tell her that he was in love with her. And his girlfriend immediately puts on an act as if nothing untoward has happened and continues to deceive him. His faith and trust in both his girlfriend and his co-worker senpai has been shattered and the current situation, in the midst of a group holiday, doesn't make it any easier to contend with. Unfortunately, Igarashi wasn't able to recognize the warning signs that something was amiss, like when he caught them alone at the festival, or when Ookuma asked him if Chiyo had said anything about him, or that Chiyo had seemed to be curiously and inordinately obsessed about what Ookuma did with Nagatsuki on their dates to the extent that she insisted on trailing them.
And, yeah, Chiyo can't be bothered to notice if her boyfriend is worried about something. She's too busy putting up a false front to maintain the façade of not knowing Ookuma from a hill of beans and never having had anything to do with him in the past.