Watashi No Hara No Naka No Bakemono summary:

Awaken the "malice" sleeping inside you.Chinatsu is a middle school girl who's an honor student and highly trusted by her teacher. She gets along with her friends, and she spends her days happily without hating anyone. However, on her period, Chinatsu realized it. That she hated her homeroom advisor, and she's extremely irritated by her surroundings. She had put a lid on her own feelings and pretended not to notice, but when she became aware of it, the "monster" inside of her begins to rampage and reveal her true nature.(Source: Comic-Days)