What Causes Light Spotting After Having Safe Sex?

Hi I am twenty years old and I had sex with my boyfriend about 3 days ago. We used a condom and I don't think anything went wrong. I am also on birth control but when we had sex I was on the last day of my placebo pill for that month. the next day I resumed the pill as normal and today (3 days later) noticed some very light spotting. Should I be worried?