What Causes Delayed Periods Post Intake Of Levonelle?

i take modafinil for narcolepsy. around 9 weeks ago i took levonelle as emergency contraception within 24 hours of having sex. since then i have not had a period at all and i have never ever missed a period from the age of 16. i have done several pregnancy tests which all come out negative. what could be the cause of my amenorrhea