Postcoital dysphoria can be defined as sadness after having sex. While most people are happy after they had sex with someone, those who have postcoital dysphoria can become quite depressed. Actually you can experience irritability, extreme sadness, melancholy, anxiety, and even shame or guilt. It seems that 32.9 percent of women had this issue at one point in their life and 10 percent of those declared that they have been struggling with the issue many times or most of the time in their life. However, when a person had postcoytal dysphoria all their life it could be a sign of childhood abuse or a personality disorder.
It seems that this problem is common in both sexes (while in some sources it is stated that it can be more common in men than in women) and it may occur more when engaging in adulterous sex or sexual intercourse with a prostitute. Other factors may be: hormonal shift after sex, unresolved issues with the partner, stress, general anxiety, or depression. Even the fear of getting a sexual transmitted disease could contribute to postcoital dysphoria.
If you experience postcoital dysphoria you should discuss your feelings with your partner; if you don't have a partner or you have multiple partners, you should at least talk with a psychologist and see how, when and why your postcoital dysphoria issue started.