Plan b . does it work . help pleaseee?

My very first time having sex I had unprotected sex with this guy . I was really scared so I had my cousin buy me a plan b pill . I took it within twelve hours . I am so scared . I took one pill since there was only one pill . I was coming on my period when I had sex . Is there a chance I could be pregnant ? Should I take a test after two weeks ? I don't know what to do . Please help .