Hi there. Welcome. Well, emergency contraceptives work by inundating your system with a big burst of hormones. That's how they prevent pregnancy from happening. They are effective and would very much doubt you are pregnant if you took it within 72 hours of the episode. Being late and irregular is VERY common after taking EC's. You will not know exactly when you ovulate for the next couple of months so should definitely use reliable birth control. You should go back to normal in a couple of months. good luck