What Causes Spotting?

Hi I am 22 years olds and my menstral started on the 20th and ended on the 24th of april, my bf and i had sexual intercourse on the 29th and we used protection but to be safe i took the plan b pill and 5 days later i start having a brown discharge and it has been going on for 2 1/2 days now i dont know if to be worried or not, or what to do?