I'm on BC (2 years) My period is in 1 week. Yesterday, I bled once & stopped. I had protected sex 3 weeks ago. Possible pregnancy? Should I be worried

No & no. Irregular menses is very common in young women. Hormonal birth control can help regulate one's periods. Very unlikely to be pregnant if taking pills daily as directed w/o fail and especially if engaging in protected sex which also lowers risk for sexually transmitted diseases/infections (although not 100% risk free). See FamilyDoc, GYN or Planned Parenthood if still concerned.