I and usually very regular on my period getting it within the same 2-3 days every month for about 5 days

I was supposed to b due around the 13th which was 2 days ago and I had sex last night and the condom broke

 I took plan b within 40 min after having sex and he finished in me The reason I think I was late to begin with was because I was stressed out due to testing I had this past week and I have been going to the gym everyday

Everything I've looked up says u should wait to get your next period in order to tell if it has worked, but I don't know if I should be expecting the one that I should be on or next month. It also stated that if you don't see results in 3 weeks you should take a pregnancy test, but my next period isn't due for about a month

Please ease my mind I'm extremely paranoid and really don't know what to do