What Causes Spotting Like Periods After Having Unsafe Sex?

Hi i had sex on last month 27th and that time the condom broke.So i took a plan B within 5 hours of the intercourse.And this month i got my periods on 18th, two days delayed(the due date was on 16th). But the period is very light and lasted only for some 2 days and the bleeding was not as usual! Any chances that i am still pregnant? Can anyone explain me the difference between menstrual cycle and implant bleeding?