Irregular Period

I have been on birth control for about a year, and I generally take it on time and hardly ever miss. My boyfriend and I had unprotected sex using the pull out method a week before my period should start. I took the rest of my pills like normal. But when I started taking the sugar pills and my period should have started, I only got very light spotting that's brown in color and sort of sticky. My period should have started Sunday/Monday and it's Wednesday and all I've gotten is the brown spotting. Does this mean I am pregnant or could it be something else?