Ovulation usually occurs 14 days after a period (if your periods are regular every
28-30 days), so chances of getting pregnant after unprotected sex Sept 22 or 23 are pretty slim, even more slim after Plan B. Urine pregnancy tests are very accurate, but if you are still concerned you can repeat it or get a blood pregnancy test. As for the late period, since it sounds like this is the first time you have had a late period, it may just be stress related.