The folks who said that ovulation happens about 14 days before your period starts are correct! I used to work a sex ed hotline and this was our #1 question.
When you get stressed out and your period is late, it's not late on days from ovulation to period, it gets longer on days before you ovulate, which is why stating ovulation as days after your period stops is inaccurate.
Essentially, if you don't want to get pregnant, you should take the time to learn about how your body works, learn about birth control, pick a form that works for you and your lifestyle and then use it.