What Causes Light Bleeding During Period After Delay?

my last perid was feb. 1, '11, i just had brown spotting yesterday, 6 weeks and 3days after my last period, 2 weeks late. today i am having a little more blood that is now dark red mixed with brown. it is enough i need a tampon, but not bleeding through. im also having a little cramping, no breast tenderness. and there was no sign of me starting my period. usualy i get breast tenderness and cramping about a week or so before i start. is it my period or am i/wasi pregnant?