Postcoital dysphoria (PCD) — unexpected sadness, irritability, or tears after sex, even if the experience was pleasurable — is common and has a specific physiological explanation.
Statistics According to a 2015 Australian study, approximately 46% of respondents had experienced this condition at least once in their lives — it's much more common than people think.
Hormonal explanation: The sharp drop in hormones (dopamine, oxytocin) after orgasm temporarily throws the nervous system into an "imbalance" — this is a physiological "downfall," not an emotional problem with the relationship.
When it needs attention If it recurs frequently and intensely, or causes episodes of deep anxiety/depression, it's worth discussing it with a gynecologist or therapist — it may be related to deeper psychological or hormonal factors.
Partner's reaction The most important thing is for your partner not to take it personally — a calm, non-judgmental reaction and affection will help this situation pass quickly.