Plan B Problems

So the last time I had sex was a month ago. My last period ended the day before that. Protection wasn’t used against my better judgement (it was stupid, I know) and he ended up finishing inside me. I took a Plan B less than 12 hours after this. The week after, I spotted for a few days. Now I’m a week late for my next period. Could this be a side effect of the Plan B or should I take a pregnancy test?