Ive seen it happen many times, recently mama~love (she is currently posting in the december DDC) had it happen that she had what seemed like a totally normal period and still got a bfp a few days later.
That said, a light period isnt reason to believe you are pregnant... go ahead and test, it will ease your mind, but we do randomly get light periods now and then even when we usually have heavy ones. The majority of the women I find who get a positive after a period had what seemed like totally normal periods.
Artist, photographer, stay-at-home-mom and Marine wife. Mom to 4; a boy and three little girls.