I'm curious as to why you would take plan B if you were using birth control regularly. If you were on birth control and had unprotected sex you should be okay. As long as it was not your very first pack; if it was your very first pack, then you need to wait 1 week for the pill to take effect if it wasn't started on the first day of your period. It's also important that you use your birth control correctly (taking it everyday at the same time and don't miss pills).
So if you were taking Plan B in addition to being on birth control you should be fine. Most people's cycles can get disrupted after using Plan B because it has a very high hormonal dosage. So everyone reacts differently to it.
If you were using Plan B because you had stopped taking your birth control, keeping in mind that you are still protected during your sugar pill week, you should still be okay since you took the pill within 72hrs of unprotected sex. However the longer you wait the less effective the pill will be.
So just to ease your mind, take a pregnancy test 10-14 days after you had unprotected sex (it takes that long of a fertilized egg to implant and for HCG levels to start rising) and see what the results are. And then take another one an additional week later or more if you still want to double check.