Should I Take Emergency Contraceptive After Unprotected Intercourse?

hi...i m a med student.so i decided to follow d rhythm method...i had sex on d 9th day f my cycle.my shortest cycle is 28 days.also since v used coitus interruptus i m sure there was no semen inside o even on my external genitalias..this happened 24 hours ago.do i need emergency contraceptive?