A.S.
It's possible that the embryo is burrowing deeper into your uterine lining and causing some bleeding... it could just be spotting for no reason. I spotted for a few weeks in my pregnancy. It was very light though. My doctor told me it was OK as long as I didn't bleed through an entire pad. You may also spot if you've had intercourse because of the extra blood down there.
If it's just spotting and it doesn't go on for too long, I think you're OK. However, if it keeps up, if it's accompanied with cramps, if it's heavy or gets heavier, you definitely need to call your doctor.
It wouldn't hurt to call your doctor anyway, to put your mind at ease.