Is the 'withdrawal' method, effective enough to prevent pregnancy, or should i take an Emergency Contraceptive Pill?

Rhiannon
After having Unprotected sex yesterday, im feeling nervous and a little anxious that only the withdrawal method was used as a method of birth control.... (not usually how i roll..).
Im wondering if i should use an emergency Contraceptive from a pharmacy, however i have taken it before and it made me feel quite Ill and left me with shitty PMS like symptoms.
Smart thing to do is to get the Contraceptive... right?