Kokoro Connect Episode 11

"A Story That Began As We Realized It"

Synopsis
On the day before Winter break, following a brief period of peace after the unleashing of desires, a new phenomenom takes place in which Yui and Iori temporarily revert to a younger age during 12pm and 5pm that day. After returning back to their normal age, they lose memory of what happened, but remember things from when they were that age. Later that night, a second Heartseed appears before Taichi through his sister's body, tasking him with watching over the others and warning him not to tell the others about his existence. The next day, Taichi stumbles upon Yui who is argued at by an old karate rival, Chinatsu Mihashi, over breaking a certain promise which she can't remember. As the gang meet up at an abandoned building, this time Himeko and Yoshifumi shrink, with the now ten-year old Yoshifumi likening Yui to a girl named Nana Nishio. After reverting to his normal age, Yoshifumi explains Nana was a girl he went out with once before she moved, leading him to be conflicted over how his feelings for both her and Yui stand. The next day, as Taichi and Himeko watch over a 14-year old Iori, Himeko voices her worries.

Source: http://en.wikipedia.org/wiki/Kokoro_Connect#Anime