What Causes Missed Period With Only Light Spotting During Periods?

Im 20 yrs old, last april 23 i got my period and may 2 we've intercourse with my bf and suddenly my period lasts until may 6(spotting only may 2-6)..is it normal? and last may 16 we have another intercourse.. and my bf used condom. im worried because i dont have menstrual period for the month of may... i took a pregnancy test but its negative.. am i pregnant?