What Causes Bleeding After Taking Postinor?

i am 27years old. i took postinor2 pills on 26th May, and i had finished my menstrual periods 2days before. 2days after the intake of postinor i started bleeding upto now,it's light bleeding but its becoming uncomfortable.I drink green tea daily(about 2-3 cups daily). Am asking if green tea could have reacted with the pills and thus the bleeding or something else is the cause?