Brown period after plan b

IM SO SORRY I KNOW THIS IS EXTREMELY GROSS BUT I NEED HELP. I had unprotected sex 3 days after my period (6/16) but I took a plan b pill and today when I woke up (6/22) I noticed that my period came 2 weeks or more early. And through out the day I've been having this brown thing ...WHAT DOES THIS MEAN??? 
AM I SUPPOSED TO BE WORRIED??!? Help