Had unprotected sex while i was fertile on 4/20. took plan b 3 hours later. within a week i was spotting. still haven't gotten period.should i worry?

Yes . . .: I would recommend worrying about sexually transmitted diseases/infections. But if you took emergency contraception as instructed within allotted time frame, then pregnancy shouldn't be an issue. In fact, young women are notorious for irregular menses so birth control pills often prescribed to regulate menses. If that isn't enough reassurance, take home pregnancy test if/when 4wks late.