My period has never been normal,but use a period tracker app to keep track, i had sex one day after ovulation, and took plan b 72 hours after that?

Usually effective: check a pregnancy test if your period is late and start using contraception!

Sooner: It should be 87-89% effective in preventing pregnancy. It is 95% effective if taken within 24 hr. after unprotected intercourse so the sooner you take it the better it works. Hope this helps. Try mot to put yourself in this stressful situation again by starting a more reliable method of contraception. Best wishes.