Late period???

I took the plan b pill on December 11 after having unprotected sex the evening before (but I'm sure that I took it within 24 hours) and around December 18-20 I had light bleeding/spotting and when I asked another person on the app she said that it was just the effects of the plan b pill but now I've missed my period by around 2 days (it was supposedly due on the 29). Is this normal and just the pill doing its job and when should I take a pregnancy test??