What Causes Cramping And Spotting After The Menstruation?

I had sex on the 1st of Oct., and started my period on Oct. 10, 2011. It normally stays on for a week but this month only stayed on four days. On the 19th I began spotting accompanied with light cramping the next day. The bleeding continued for three days than stop. However the light cramping comes and go. I took a pregnancy test on the 21st, but it came out negative. Could their still be a possibility that I am pregnant or could this be the health problem?