Once those cells start growing where they shouldn't, it's called endometriosis. But many women don't notice symptoms for years. The abnormal endometrial cells don't exit during your period — in fact, they don't leave your body at all. They stick around, and they multiply every month when your hormones are stimulated again. These cells continue to multiply, and it can take a few years before you experience any symptoms. As the long-term cycle continues, you may eventually experience scarring (called adhesions) and pain. That's when you realize something's wrong.