Plan B works pretty well on the whole. This likely prevented you from conceiving. But it works because it is a big dose of hormones. Those hormones interrupt things so conception can't occur. However, those same hormones cause us to be irregular. Spotting, late or missed periods, heavier than usual bleeding all can happen from the hormones left over from plan B. This lasts usually for 2 to 3 months. You can rule out pregnancy if you feel you need to but would be absolutely shocked if you were pregnant. let us know how you are doing!
Since Plan B says in its material that it only works 7 times out of 8, I would go ahead and do a pregnancy test, but probably it worked and all you are dealing with is that it makes your cycle wonky for a month or two.