Its best taken within 72 hours, but can be effective up to 120 hours after sex. Since you took Plan B within 72 hours, you have a higher chance of preventing pregnancy.
Yeah, it is still possible that you become pregnant, but its not a very high chance. Just relax and wait for your period, if it doesn't come then take a pregnancy test. Plan B can cause your cycle to be a little wacky for awhile so it may be normal for you to be early or late. You may also have spotting between periods and irregular periods for up to three month after.
Don't stress about symptoms or anything like that since the side effects to Plan B are very similar to pregnancy symptoms.