What Are The Chances Of Pregnancy Despite Taking I Pill?

i had my last periods on 01.12.2014. on 31.12.2014 i had unprotected sex with my husband. i tool i pill with in 12 hours too.. on 20.1.2015 i hv 2 or 3 spots only after thati dont hv my periods.. is there any chances of pragnancy . what are tge reasons i dont hv my proper periods till day. and what will be reason for those spots.