What Causes Irregular Vaginal Spotting After Taking Plan-B Followed By Unprotected Sex?

I had unprotected sex but took a plan B pill the following morning. Three says latter I started spotting. Spotting lasted for 3 to 4 days and then stopped. About a week after that I started bleeding. I thought it was my period but it only lasted one day. I have not started my period but I have never experienced anything like this. My periods and regular always on time. Is what I have experienced normal? Am I pregnant?