Can Pregnancy Occur Despite Taking Morning After Pill?

my girlfriend and i had unprotected sex one night but we stopped in the middle of it because we decided to wait until we had protection. the next day she told me she had read online that there is something called pre cum that happens during sex on my part and that it could lead to pregnancy. to be sure the next day we bought plan b and she took it immediatly. now a couple weeks later she has her period early and she says it isnt her period. she explains to me that it isnt how she usually has it and that there is less bleeding. she describes it as spotting of a darker blood color. my question is if this is normal because of the plan b? or is it possible that she might be pregnant.
thanks for the help :)