Skirt no Naka wa Kedamono Deshita.

The story begins when the reserved Shizuka attends a local mixer party but has trouble adjusting to the atmosphere. A beautiful older female college student named Ryou starts talking to her, and the pair hit it off. Shizuka goes to spend the night at Ryou's place. However, Ryou unexpectedly kisses Shizuka and pushes her down. Shizuka thinks the beautiful woman is a lesbian but soon discovers that Ryou is actually a man dressed as a woman.

(Source: Anime News Network)