I had my period for the first 5 momths with my daughter. It is because your body doesn't produce enough hcg to stop the period.
I had my period for the whole pregnancy with my second child... It was weird but doc said is normal...
I know someone who had their normal heavy period every month while she was preg
Light spotting around the time you should start your period could just be implantation bleeding, it could be heavy enough that it could be mistaken for your actual cycle.