What Causes Light Spotting During Periods?

My boyfriend and I had sex on Feb. 20th, and the condom broke so I took the morning after pill the day after. My period comes at the beginning of the month, so when it came on March 4th, I assumed that was my period, but it was pretty light and was a brownish color. Right now, April 10th, my period is a week late and I'm getting worried. I took a pregnancy test yesterday that came out negative...should I still be worried?