Am I Pregnant?

I am on birth control. My boyfriend and I have sex all the time. I was three days late on my period and now I'm having light spotting that's brown and not enough to get on the tiolet paper when i wipe. I'm not sure if that means something or I'm just being silly. I took a pregnancy test a few days ago and it came back negative but I usually have a lot more blood then this and I have no cramps. Please help.