Yes, irregular menstrual cycles are a side effect. Here's some information on side effects:
http://www.crisispregcentre.org/education/the-morning-after-pill-or-plan-b/
Your period should be back to normal within a month. If longer than that, you should call your gynecologist and see if you should come in for an exam and testing. But irregular bleeding is a common side effect, so it should return to normal soon.
Hi there and welcome. Yes, plan B can cause bleeding like this. If it goes on for a long time or becomes heavy, call your doctor. It's basically a hormone and you've jolted your hormonal system. Now you are out of sync and it will take a while to get back to normal. good luck