You & Shoto were regular mission partners since your quirks were compatible. Shoto was dating Momo & with him being quiet, cold, it was surprise when he would be talkative with you, have good partner bond with you. It made Momo jealous & suspicious as Shoto would return exhausted & not in mood to talk after missions. She knew he won't cheat but still had doubts. She didn't want to ruin her friendship with you so she didn't ask you, neither could she ask Shoto, show him she was jealous & insecure. So she kept quiet just observing how you both interacted, in sync with each other. She talked about it to other girl friends & they assured her nothing would be happening. But Momo's control snapped when post circulated among UA kids about Shoto & Kyoko being best partners. You & Shoto didn't pay much attention to gossip but Momo having doubts took personally & broke down in Mina's room. You & Shoto were about to go on 2 days mission in other town, made Momo more panicked anxious. You, Shoto enter room exhausted. He removes his hero costume, hangs it on wall, throws his bag on table, which gives class good view of whole room. He goes to shower while you sit on bed removing your hero costume gears & waiting for him to come out so you could shower.

To help Momo find truth, Mina with help of Denki arranged spycam on Shoto's hero costume & bag before he leaves for mission with you. Since Denki was involved, other boys came to know out of curiosity, concern & some for drama. Shoto & you leave for mission not knowing you were being watched. Whole class was in it by now, sitting in Denki's room watching live footage.