It is possible that Plan B is going to cause another period--because as Wonder4 notes above, it contains
progestins. And the symptoms you are experiencing could be PMS--again. We often use
progestins to induce a period when a woman has missed hers. It is very common for Plan B to cause irregular bleeding, even into the next cycle. If you have any doubts or worries, take a pregnancy test, but it sounds like you should be OK.
Good luck!