There is often a comedown that happens after having sex. Your heart rate slows, your breathing gets easier, and your muscles relax. For some, this is nothing more than a cue to take a nap. But for others, this postcoital lull leads to something more serious — deep, hard-to-describe melancholy that can last anywhere from a few minutes to several hours.
Postcoital tristesse is defined as feelings of sadness, anxiety, or depression after sex. It happens even if the sex is good, the sex is consensual, and the relationship is strong, although it can certainly also happen when any of those things aren't true. Some people break down into tears after sex, some just feel hollow or sad. Some people get argumentative with their partner. There is often a feeling of let down.
Because people experience PCD differently, it can be hard to describe its symptoms. But, in general, it can be thought of as experiencing a negative emotion that bears a harsh contrast with what you were feeling a few minutes before.