Twisted (movie, 2004)

Recently promoted and transferred to the homicide division, Inspector Jessica Shepard feels pressure to prove herself -- and what better way than by solving San Francisco's latest murder? However, as Shepard and her partner, Mike Delmarco, soon discover, the victim shared a romantic connection to her. As more of Shepard's ex-lovers turn up dead, her mind starts to become unstable, and she begins to wonder if she could be the very killer she's trying to track down.