What Causes Light Spotting Between Periods?

I was suppose to start my Period on Sunday which i did but it was only a dark brown dried blood. On my pad ans tampon there is nothing but a little brown blood here and there. I have still not had a period only this brown dried blood, I am on a birthcontrol called Orthotricyclen_Lo and have been since Feb. My periods have always been regular and are usually heavy, I am sexually active and we do not use any form of protection other than my birthcontrol and the pull out method. Could i be Pregnant?