What Are The Side Effects Of Plan B Ipills?

Hi, my girlfriend took "Plan B" 3 hours after an accident had happened. She does not start her period until about the first week of each month. She has felt really tired and has been cramping, but the cramps have been letting up a little bit she says. I think this is a sign that she is about to start her period. I guess my question is, how effective is the Plan B pill and are her chances of being pregnant high or low?