is possible only,if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period.
Sometimes precum or prostatic fluid,secreted prior to ejaculation,causes pregnancy in low percentage if it takes place inside vagina.
But in your case,chance of pregnancy is almost nil and you also took Postinor tab for safety.
Don't be much worried and wait for the next period. If it not comes,do a urine test for hcg
after 1 wk to clear doubt of pregnancy.
Practice safe sex
by using safe period/OCP/Condom etc to avoid unwanted pregnancy. Be well.