Confused..

Ok I never heard this before but I just want to make sure. I had sex on the 15th of February. Condom broke. Finished inside of me. So I immediately took plan b. I had just started my period today (the 21st of February). And I was excited and assumed that I was in the clear of being pregnant. But this girl told me it didn't matter if I had it. I won't know for sure till I do or do not get my period next month. Is this true?