I can tell you that women DO become pregnant using emergency contraceptives. They are not as effective as daily birth control. You can find some posts on this website.
Taken within 72 hours it is effective and your chance are low.
You had sex on day 8 or your cycle. Normally you won't ovulate until sometime between days 11 and 16. Sperm can survive about 5 days. So, if you ovulate after day 13, it is very unlikely you'd get pregnant.
Plan B does have side effects. Your next SEVERAL periods can be early or late AND heavier or lighter than normal. Don't worry if you are late, take a home test.
Hope it helps. Good luck.