A lot of factors - should I take a pregnancy test?

So, I've had a sinus infection and have been on antibiotics. I know being sick and antibiotics are both things that can delay your period. But here's the thing - when I was ovulating, my fiancé and I had sex and the condom broke. When I went to get Plan B, the pharmacist told me it might not work because antibiotics can make BC less effective.
Currently, my period is 4 days late. Should I be worried? Should I take a pregnancy test? The day my period was supposed to start (Dec 4) I had a little spotting, but then nothing more. Advice?