Covering the opening of the vagina, the hymen is a thin layer of skin that can be torn or stretched open with penetration. Found about half an inch inside the vagina, the hymen has tiny holes and a small opening in the centre that allows for menstrual flow. Sometimes the tearing of the hymen can be uncomfortable and cause bleeding. But other times, you might not even notice when it happens. Masturbation , using tampons, and even rigorous activity like horse riding, dancing, riding a bike, and other sports may stretch the hymen before vaginal sex and can cause rupture or tearing.