I'm currently 25 weeks pregnant after taking plan B so it isn't always effective. However, when you do take it it does mess up your cycle. The pharmacist told me when I bought it that I should expect my period to be 1-2 week late and if I was more than 3 week late to take a pregnancy test.
Plan b is only 80% effective not a reliable form of birth control at all so you could be pregnant. If you luck out and are not it will mess your cycles up for a bit. Also you are only supposed to use plan b 2-3 times per year so if your not pregnant and want to continue to have sex best get on the pill or another reliable form of birth control. Good luck to you. :)
THE SOONER YOU TAKE IT THE MORE AFFECTIVE IT IS
It could irregulate your cycle but I will say, not to scare you, that I took the plan b pill after two days of unprotected sex and I am currently 29+2 weeks pregnant.
WHEN U TAKE THAT IT MESSES UP YOUR PERIOD!