Responses
Chiyo is unable to believe what Igarashi says because she, herself, cannot be trusted to be open and honest. She has been lying to and deceiving Igarashi about her "relationship" with Ookuma for over 40 chapters now. Those who lie to their significant others (Chiyo) tend to have less faith and trust in their partner (Igarashi).

Chiyo pisses me off!!!!! Like WTF?! If my boyfriend comes to me and says ,,I helped a drunk senpai and went to karaoke with girls just because I couldn't leave him. Bur nothing happened. " I will believe him and say it's ok since nothing really happened. She doesn't trust him even though he is so faithful and patient about having sex! She always says NO but in the end it's his fault.. (╬ ̄皿 ̄)凸(⊙…⊙ )