My period started the 25th or may and ended the 28th. i had unprotected sex the 29th and took plan b right afterwards on the same day. however, today on june 5th, i am starting to spot a dark almost rust color. should i be worried that i'm pregnant?

Plan B alters hormonal status and bleeding is due to fall in hormones. You may do home pregnancy test 3 weeks after the sex episode, if the result is negative, repeat the test 4-5 days later, unless you get a period by then. Use first morning urine for the tests. Use contraceptive all the time, every time. Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guideline