Could I have been pregnant before taking the plan b?

I posted on a different app about my pregnancy test and how the results came up super quick. The test line was super dark and the control line was super faint. I took the plan b pill within 3 hours after having sex. I was told that I was probably pregnant before taking the pill. The last time I had sex was Sept 16th and I had my period from Sept 30-Oct.5. I couldn't have gotten pregnant from Sept 16th because I got my normal period that month. I would have gotten pregnant in the beginning of October and that the pill didn't work for me.

Am I right? I doubt they could be right because you can't get your period when you're pregnant.