"Postcoital dysphoria (PCD) is the experience of negative affect following otherwise satisfactory sexual intercourse," a team of researchers explained in a 2011 study published in the International Journal of Sexual Health. "Under normal circumstances the resolution phase of sexual activity elicits sensations of well-being along with psychological and physical relaxation. However, individuals who experience PCD may express their immediate feelings after sexual intercourse in terms of melancholy, tearfulness, anxiety, irritability, or psychomotor agitation."

Importantly, PCD refers to when there is no discernible reason for the person to feel negatively about the sexual experience that just happened—it was consensual, pleasurable, and perhaps even induced some orgasms, and yet the person still feels upset afterward without a clear understanding as to why they're feeling that way. It can happen to someone even when the person they slept with is someone they're in a serious, committed, and loving relationship with, just as easily as it could happen when it's with a first-time or casual partner.

There has yet to be much substantive research done on PCD, and so it's still not a well-understood phenomenon even among sexual health professionals.

"We unfortunately don't really understand postcoital dysphoria very well," Vanessa Marin, a psychotherapist specializing in sex therapy, tells mbg. "We really only know that it exists. It doesn't seem to have any relationship with the type or quality of sex that you have, or your relationship with your partner."

The few studies that have been done show that PCD is a fairly common experience: A 2015 study found 46 percent of straight women had experienced it at least once in their life, and 5 percent had experienced it a few times in the last four weeks. Another study released last month found 41 percent of men (most of whom were straight) experienced PCD at least once, and 20 percent had experienced it in the last four weeks. (Side by side, these two studies suggests PCD happens at fairly similar rates between men and women, but the latter study actually found women were about twice as likely to have experienced PCD in the last four weeks compared to men and nearly three times as likely to have experienced PCD in their lifetime.)