"Tree and Itsuki's Transformation Diary" (Note: "TS" typically stands for "transformation" or "gender transformation" in Japanese manga/light novel contexts)

Koyanaigi Itsuki, a slightly small-framed, feminine-faced third-year middle school student, collapses with a mysterious high fever on the day of his middle school graduation ceremony. After three days and three nights of delirious fever, when the fever breaks and he wakes up... For some reason, his body has become a girl's! Confused by suddenly becoming a girl, Itsuki recalls a strange dream he had while delirious. In that dream, he was not male but... This is a story of Koyanaagi Itsuki, who suddenly became a girl one day, and the tales of his other cheerful companions...