Did The Plan B Alter Her The Menstrual Cycle?

My girlfriend and I had a "mishap" almost 3 weeks ago. She took the Plan B pill within 12 hours, and everything seemed fine. A few mornings ago, she woke up with a sharp pain in her stomach and bleeding. It has continued for 3 straight days now. I know she is within two weeks of her monthly cycle. Did the Plan B alter her cycle? Did it come early, or could it be something else?