What causes bleeding after sex. The walls and opening of your vagina can also get injuredcausing bleeding. 10.03.2018 Faegul Knee osteoarthritis: Some sexually transmitted diseases have the side effect of potential vaginal bleeding after sex — along with a slew of other symptoms. Endometriosis may obstruct the tube and keep the egg and sperm from uniting. Your privacy is important to us. Rarely, cervical cancer can also cause postcoital bleeding. Learn more about it here. An IUD can sometimes become displaced. Because your symptoms are related to sexual activity, your doctor may also ask about your sexual history.