Timing is Everything

A 35 year old woman presents with pleuritic chest pain and dyspnea. Her X-ray is shown above; the diagnosis is easy. But as you are about to discharge her, you see something interesting in her medical chart. She has presented with the same diagnosis every month for the last six months. Though the idea of Munchausen's or factitious disorder crosses your mind, you realize that the diagnosis was confirmed every time. She also denies self-inflicting this. As you focus on the past medical history, you realize she's gotten this problem every 28 days.

Challenge: What's the diagnosis?

Image shown under fair use.

*Note: I'm on vacation Monday, so the next case will be next Wednesday.