Thanks for the query.
As you took the pill within 72 hours, there is less possibility of pregnancy
Emergency pill can alter the menstrual cycle as a side effect. So the bleeding you are having could be a combined effect of emergency pill and progesterone
As the bleeding is not heavy no need to worry.
Better to take treatment for irregular periods
to avoid future problems of infertility
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429