What Causes Irregular And Persistent Brown Spotting?

Hi, So my boyfriend and I had sex on 9-25-10. We used a condom and he pulled out, but i was a little worried because we were messing around before he put the condom on, so I took the plan b pill that next day. A week later on 10-4-10 i started spotting. It brownish most of the time, and occasionally red. It is still going on and its 10-12-10. Is this normal? SHould i be worried about this? My normal period is due to start on the 14th of this month.