For lots of people, sex brings feelings of relaxation and even pain relief, but some people experience a "period of intense depression and irritability after an orgasm."
Dr. Richard Friedman writes in the Times that he initially thought this postcoital depression had a deep-seated cause, perhaps "hidden conflicts about sex" or "ambivalent feelings" toward a partner. But he found that, in several cases, the problem seemed to exist entirely on its own, perhaps as a result of a rebound in the amygdala, the brain's center of fear and anxiety, whose activity is dampened during sex. The solution: antidepressants, which made sex less pleasurable for the affected patients, but also reduced the postcoital "crash." [NYT]