I have my period Nov. 14 to Nov. 18 and have a sex with my boyfriend on the 7 day of my period (Nov. 20), and we used withdrawal method that time, am I safe for sex that time? How could I know that I am fertile?

Yes it is possible to get pregnant during your period, You will have to track and know your most fertile times which differs for most women.

Withdraw is not a method, it is a way to become pregnant since it only lowers the amount of sperm, not stop them.