Birth control could be a reason for intermittent bleeding
after sex. The various other causes for vaginal bleeding
after sex are injury and friction (this you rule out), STDs like Chlamydia, gonorrhea, and trichomoniasis, cervical polyps, yeast infections, non specific vaginitis, endometriosis, cervical dysplasia which is a precancerous condition, uterine polyps, uterine fibroids and cervicitis. None of them are conditions that should be ignored.