Yandere Ouji ga Shachiku Onna no Watashi wo Hanasanai

The main character is a busy modern-day corporate slave. One day on her way home late from overtime work, she fainted, and woke up to find herself tripping into another world. She was sheltered by a kind prince, but as peaceful days passed by in his palace, she began to feel a little uneasy about the prince and the people around him. One day, while searching for a way to return to her original world, she found a picture of the prince and a woman who looked just like her...?
(Source: DLSite, translated)