My period came on the 13 and finished the 17. i had unprotected sex on the 19 (2 days after). i had a plan b within 24 hours. a week later, i had spotting that lasted for 3 days, around the time i would be ovulating. is pregnancy likely?

Unlikely, but...: Plan B can alter the menstrual period for the cycle in which Plan B was taken, and sometimes the next cycle too. One should be sure to check a home pregnancy test about 3 weeks after sex, because Plan B works most of the time, but not all the time. One should be sure to start a form of reliable birth control if planning to have sex again, so that Plan B will never be needed in the future.