I've been told that when the egg embeds itself into the uterus you have slight spotting...i noticed this with my current pregnancy but didn't with my first two as they were unplanned and we weren't trying. I think it you are fine but if you continue to bleed you may not be pregnant...but it isn't uncommon for some women to have light bleeding throughout their pregnancy that they mistake for a light period but isnt.
Also I already have a 7 and 3 year old and had a miscarriage last year.