her period is to be in 31st of January but it's feb 10th and her period doesn't start yet

hi, we had a sex once without protection so she took ipill within 24 hrs and she had her period.after that we had a sex with protection.her period is to be in 31st of January but its feb 10 and her period doesnot start yet.Is this because of ipill or is she pregnant? please reply as soon as possible.