Thanks for using HCM.
Generally in a 28 day long cycle ovulation (ovum formation in your ovary) occurs near the 14th day, and ovum remain viable for 1 to 2 days where as sperm can remain viable for 2 to 3 days so you may be pregnant in 18th day.
So you should use some emergency contraceptive
pills which act in post sex period and prevent pregnancy
in 99% cases.
It acts by-
1) Preventing the fertilisation( meting of sperm with the ovum).
2) It prevents implantation
(prevent attachment of the egg to your uterine wall).
if already fertilisation completed.
3) It interfers the function of corpus luteum
which helps inmaintaining pregnancy.(If already Implantation completed)
so it acts via 3 mechanism failure rate is very low.
But for its action you have to take it within 72 hours and strictly 72 hours. More earlier the intake more efficient will be the action. But not effective after 72 hours.
You can get this in any medicine shop.
Best of luck.