hi, thank you for your query. this is common after effect seen in people taking birth control pills.many people report abnormal residual bleeding after sex or when aroused. The brown discharge is just old blood from your menstrual. sometimes, although the menstrual cycle finishes, blood still gets stuck in the uterus, and when your aroused and stimulated it causes your uterus to contract which forces out the old blood trapped. It s nothing to worry about. some other Causes can be hormonal derangements, thyroid hormonal imbalances, endometritis, endometrial hyperplasia, pregnancy etc. You should consider visiting your doctor/gynecologist at the earliest for thorough evaluation and further investigations (hormonal and blood work-up along with ultrasound scan) to look for the underlying cause. and also for change in the birth pills. thank you, wishing you good health