Irregular bleeding due to pill or something else?

Okay, I just started the pill a month ago. My period started on the 23 of may and has been over for about 5 or 6 days now. The other night after sex I noticed some very light bleeding. Yesterday was the first day of the placebo pill, and tonight I'm having more light bleeding. Is my period coming again because of the placebo pill, is this just spotting because of the pill, or should I be more concerned?