Kuso Onna ni Sachi Are

"I've been cheating on you. Oh, and it's not just one guy. It's three. Sorry about that." Back in middle school, Sunao Akiyoshi's first-ever girlfriend shattered his heart, and he swore off love. However, all that changes when he develops a crush on a girl in his university's film club. Just as he's finally beginning to move on from the past, he bumps into his ex-girlfriend. Little does he know that this is only the beginning... To his horror, he wakes up in her body the next morning!

(Source: MANGA Plus)