False negative HcG blood test?

I messed up my BC pill two weekends ago...I skipped three days by accident. I was away on vacation. So I knew I was supposed to throw the pack away but I decided I wanted to prevent the early period. So I doubled up to catch up. Yes I know I shouldn't have and wont ever do it again, but I did it...so I naturally spotted for a few days. I had unprotected several times with my fiance that same week, when the spotting eased up. My period was due the following week. It came but very lightly...it disappeared...then spotted, then it seemed like it came again with slight cramping..then again, it disappeared...and its been doing the vanishing and reappearing act all week with slight cramping. I did a blood test on Thursday after the Tuesday my period allegedly came, at the doctor's office because I was taking Accutane for my skin and they always do a pregnancy test and it came back negative. I am spotting on and off, and sometimes its brown and sometimes traces of red...What do you think? Did the pill mess me up because I screwed it up, or am I pregnant?