I'm on the pill (3 yrs)and I had protected sex about two weeks ago. When I started my period it was lighter than usual. Can this be a sign for anything

Bleeding. Even with perfect pill use there is a 3% chance of getting pregnant, so check a pregnancy test just to be safe. The pill sometimes lightens and may eventually stop your period completely and that can happen at anytime. It is not harmful, but again check the pregnancy test to be certain.